Flak jackets are a form of padded protective clothing designed to provide at least partial protection from things such as blunt-force trauma, shrapnel and other projectiles. This can be seen when it protected Iruka Umino from kunai and fūma shuriken while he was searching for Naruto and the Scroll of Seals and was ambushed by Mizuki. They can also prevent lethal injuries from swords and other weapons as seen during Baki's fight with Hayate Gekkō. For the most part, they have superseded the plate armour widely used by shinobi during the Warring States Period and the First Shinobi World War. They are usually worn by chūnin and jōnin over their normal clothing in most villages, although the design varies among the villages and have also changed over time. The jackets are among the most iconic pieces of shinobi equipment. Most of them have pockets or scroll pouches, which can be used to store scrolls, weapons, and supplies. This allows a ninja to access them quickly and/or quietly with only one hand, as seen with Kakashi Hatake's case when he normally does this secretly to extract a kunai.
It may also be possible that flak jackets were invented during the Warring States Period, as a member of the Hagoromo clan was seen wearing armour that closely resembled a flak jacket.
In episode 316 of Naruto: Shippūden, many of the reincarnated shinobi are stated to be genin-level. Nevertheless, nearly all of them are seen wearing flak jackets.
At least two of the Hokage (possibly three) are seen to be wearing the flak jackets.
Konohagakure (current): Konohagakure flak jackets are green. They have three scroll pouches on each side of their chest for scrolls, medicine, and other ninja tools. For protection they include a neck guard and extra padding over the shoulders (which is fastened on by steel buttons). They also have a zipper down the middle. The crest of Uzushiogakure is integrated onto the back of the jacket, symbolising the ties Konoha had with the former village.
Konohagakure (movie variant): In Naruto Shippūden 4: The Lost Tower, Minato's team was seen wearing a unique flak jacket. They are dull brown and came with angular shoulder guards, a throat guard, a neck guard, and one scroll pouch attached to the left shoulder. The padding was thinner on the sides and the front and back extend downwards. They also came with knee and elbow padding. It is possible that this was merely an experimental model as it hasn't been seen prior to or after the Third Shinobi World War.
Konohagakure (wartime variant): Worn during the Second Shinobi World War, this flack jacket consists of a steel grey over-vest worn over Konohagakure's previous flack jacket. It bore a passive resemblance to the Land of Demon's flak jacket, but lacked a neck guard. It was connected by straps over the shoulders and hips. It also came with protruding shoulder guards, extensions to protect the lower throat and abdomen, and steel gauntlets. It has also been seen being worn on its own with neither a throat guard nor protruding shoulder guards (as is the case with Kakashi Hatake). It was probably just a prototype as it was only worn by a selected few during the war.
Konohagakure (previous): This flak jacket was worn by Konoha shinobi approximately ten years prior to the start of the series during the Second Shinobi World War. It resembles the current one but lack pockets and sometimes neck guards. The lack of scroll pouches was remedied by wearing two additional equipment pouches on the lower back instead of one (as is the case in modern times).
Konoha ANBU: Konoha's Anbu flak jacket is steel grey, come with steel gauntlets, and seem to be connected with a interwoven band near the stomach along with two straps over the shoulders. They also come a sword mounted on the back just over the right shoulder by a metal holster. Furthermore, they come with two equipment pouches instead of one to compensate for the lack of scroll pouches.
Sunagakure: Sunagakure flak jackets are beige. They may or may not include shoulder padding, shoulder guards, neck guards, and scroll pouches over their stomachs. They all however are fastened by a zipper in the front and by metal clasps on the shoulders. They also come with some kind of pouch in the lower back.
Sunagakure (variant): Another flak jacket model has been seen being worn by several genin-level shinobi from Sunagakure. It is brownish-beige, fastened by a zipper, and lacks scroll pouches and reinforced shoulder padding. However, this is compensated by having a thick neck guard that envelopes the entire neck and segmented steel arm-guards.
Iwagakure (current): The current Iwagakure flak jackets are bright brown and look exactly like the old version, but instead of brown cloth, red is used in the uniform. Also, the pouches and the connecting strap are now brown rather than grey like the previous generation.
Iwagakure (previous): The old Iwagakure flak jackets are dark brown with only one sleeve on the left arm. They have a large grey zippered pouch over their stomach and a larger, buttoned one over the lower back of the vest. Both pouches are fastened on by a grey strap over the stomach. None of these flak jackets hold the presence of neck guards though they do retain shoulder padding held in place by buttons and zipper that go down the middle.
Kirigakure (current): Kirigakure's flak jacket are very basic in design. They are slate grey in colour and pocketless. The most notable features of these flak jackets are the extended shoulder guards and lower extensions on either side to protect the lower abdomen. Unlike the majority of the styles used (which use zippers) this flak jacket was fastened from the sides by straps.
Kirigakure (variant): In the anime, Team Ganryū and several other Kiri shinobi were seen wearing a variant of Kirigakure's flak jacket. It resembled Iwagakure's version but is coloured slate grey and sometimes came with neck guards.
Kirigakure (previous): In the anime, a variation on the standard Kiri flak jacket was seen. Unlike its counterparts, it came with two scroll pouches on the lower left side, a pocket on the right chest, a built-in pouch on the lower back, a neck guard and is fastened by a covered zipper. The village's symbol is also embedded on the back. However, there is only minimal padding on the sides.
Kirigakure Anbu: Kirigakure Anbu flak jackets are also slate grey in colour. They come with or without a neck-guard but have elongated padding over the shoulders, some bearing their village symbol on each padding. Like the flack jacket used by the Kirigakure's regular forces, it is held together with straps from the sides.
Kumogakure (current): Kumogakure flak jackets are white. They have a rather utilitarian version with no scroll pouches and are fastened by only one strap over the left shoulder. The strap is attached via a button and the jacket has a strange design on the front which consists of one diagonal line going down to the left with four small lines on top and one medium size line line going down to the right that is perpendicular the previous line. And while the flak jacket only covers half of the torso, this is compensated with the use of shin and arm guards.
Kumogakure (previous): This flak jacket is dark grey and was worn by Kumogakure shinobi approximately ten years prior to the start of the series. Unlike the current style it had extra shoulder, neck padding, mesh armour around the collarbone, three scroll pouches on the chest, and is fastened by metal clasps on the right side (though some don't have scroll pouches or mesh armour as is the case with the Kumo spectators).
Otogakure: Otogakure flak jackets are black. For protection, they come with shoulder padding (which is fastened on by steel buttons) and a neck guard. They are also fastened by three metal clasps on the left side and have a pouch in the front just over the stomach.
Amegakure: Amegakure flak jackets are slate grey. They cover the shoulders, torso, include a neck guard, and are held in place by a zipper. Some have been seen bearing two swirl designs on the chest. However, they do not come with scroll pouches.
Takigakure (current): In the anime, Takigakure flak jackets are sky blue and are fastened by a zipper and shoulder straps. However, they lack scroll pouches, neck guards, and reinforced shoulder padding. Some Taki shinobi wear metal shoulder-guards attached to their flak jackets.
Takigakure (variant): In the anime, a group of Taki Kunoichi wear a variant of Takigakure's flak jacket. It was light blue, bore a passive resemblance to the Konoha Anbu's uniform but has two buttoned pockets on the lower stomach. It also came with Katana holster on the back, came with wrist guards, and was worn over a short sleeved shirt with a body suit underneath.
Takigakure (variant): In the anime, a variant of Takigakure's flak jacket has been seen that is quite different from the current style used. It resemble's Konohagakure's model (complete with a neck guard, buttoned shoulder padding, scroll pouches and a zipper) but is coloured purple and lacks the spiral crest on the back.
Takigakure (previous): In the anime, this flak jacket was worn in Takigakure approximately ten years prior to the start of the series on the eve of the Second Shinobi World War. It was dark purple, and had thick shoulder padding, a neck guard, extensions on both sides to protect the lower abdomen, one large pocket on the right near the stomach, and is fastened by a zipper. However, side protection was reduced to a minimum amount of padding.
Kusagakure: In the anime, Kusagakure flak jackets are similar in style to Sunagakure's flak jacket, they are brown and are equipped with long shoulder guards, thick neck guards and several scroll pouches around the zipper that fastens them. They are also fastened by metal clasps and straps over the shoulder.
Hoshigakure: In the anime, Hoshigakure flak jackets are blue. They have two scroll pouches near the left hip and are held together by straps on the right side and by the neck-guard, which surround the entire neck.
Jōmae Village: In the anime, the Jōmae Village's flak jackets are brown, have no sleeves, and appear to be completely plain. Other than that, they come with a throat guard that surrounds the entire neck, shin guards, extend slightly downward to protect the lower stomach, and are attached by straps over the collarbone and hips. They also seem to favour using mesh armour to protect the shoulders.
Moyagakure: In the anime, the flak jacket worn by the shinobi from Moyagakure are beige in colour and had two large pockets on the chest area and two built in pockets at the sides. The shoulder padding is also reinforced and the zipper is covered. In addition, they come with elbow and knee-pads.
Benisu Island: In the anime, the flak jackets worn by the shinobi of Benisu Island are beige. They have two large pockets on either side of the chest, two in the lower sides, extra shoulder padding, and a built in neck guard. In addition, they also have a concave triangular symbol embedded in the back. They are also fastened by a zipper and can be worn over a piece of cloth (which also has the same symbol) that hangs down to the wearer's knees.
Prajñā Group: Flak jackets worn by these shinobi are dark green. Other than that, they lack any accessories except for three pouches on the lower back that are fastened by a strap over the stomach. They also possess a zipper that runs down the middle, neck guards, and padded shoulder-guards.
Fūma clan: The flak jackets worn by these shinobi are dark blue. They possess extended shoulder guards but no neck guards. They also come with black bands on the sides and are fastened by a zipper and over the top with the same black bands with metal clasps. They also come with iron gauntlets and a fūma shuriken holster on the back. Also instead of a single white equipment pouch on the lower back, there are two grey ones mounted on the hips.
Land of the Sky: In Naruto Shippūden 2: Bonds, the Land of the Sky flak jackets are purple with two basic pockets on both sides of the chest, are fastened by a covered zipper, and have a protective flap beneath to protect the groin. They also bear their village symbol integrated onto both of the shoulders which is reinforced with additional padding.
Land of Demons: In Naruto: Shippūden the Movie, the flak jackets worn by Land of Demons's border guards. They are purple or slate grey in colour. They look similar to the Kirigakure's current flak jacket, except they possess thin neck guards. Like Kirigakure's, this flak jacket was fastened from the sides by straps and are pocketless. In addition, they come with black wrist and shin guards.
Land of the Moon (Loyalists): In Naruto the Movie 3: Guardians of the Crescent Moon Kingdom, the flak jackets worn by Land of the Moon's military forces loyal to the royal family after the coup d'etat. They are black in colour and bear a vague resemblance of Moyagakure's flak jacket (minus the chest and side pockets). It appears to be fastened by a covered zipper and strap-on padding over the shoulders which also bear their coat of arms (in the blue badges) integrated onto both of the shoulders. Unlike the vests worn by the Land of the Moon's regular forces, they bear a large emblem that resemble bird-like creature (hawk or phoenix) integrated on the left side of the chest (presumed the military insignia) and come with two pockets on right hip while the left hip has a cutlass mounted on it.