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 during his search 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 For the most part, they have superseded the plate armour worn by shinobi during the Era of the Warring States and the First Shinobi World War.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.
Several genin in Sunagakure such as Matsuri and Ittetsu have been seen wearing flak jackets. This may be due to the fact that that Sunagakure has a very low population (two out of five stars) but needs to maintain a strong military to maintain its place within the Five Great Shinobi Countries.
Konohagakure (current generation): 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 (second generation): 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 on the left side. 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 (first generation): 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 as is the case with Dan Katō's. 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 (current): Konoha's ANBU flak jacket is steel grey resembles the one worn by Sunagakure (minus the steel clasps, neck and shoulder guards, as well as the scroll pouches. They do, however, come with steel gauntlets and seem to be connected with a interwoven band near the stomach along with a zipper. They also come a sword mounted on the back just over the right shoulder.
Konoha ANBU (previous): Worn during the Second Shinobi World War, this flack jacket consists of a steel grey over-vest worn over Konohagakure's first generation flack jacket. 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.
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.
Sunagakure (previous): Another flak jacket model has been seen being worn by several shinobi from Sunagakure. It is brownish-beige, is fastened by a zipper, and lacks scroll pouches and reinforced shoulder padding. However, this is compensated by having a neck guard that envelopes the entire neck and comes with segmented steel arm-guards.
Iwagakure (previous): The old Iwagakure flak jackets are dark brown with only one sleeve on the left arm. They have a large pouch over their stomach or the lower back of the vest. None of these flak jackets hold the presence of neck guards though they do retain shoulder padding held in place by buttons and zippers that go down the middle.
Iwagakure (current): The current Iwagakure flak jackets are bright brown but look exactly like the old version, but instead of brown cloth, red is used in the uniform. Also, the pouch and the connecting strap are now brown rather than grey like the previous generation.
Kirigakure: 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 side by straps.
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 also 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 only one strap over the left shoulder to hold it in place. 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.
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, side clasps, fish net armour near the collarbone and three scroll pouches on the chest (though some don't have scroll pouches or fish net armour as is the case with the Kumo spectators). They also bear a passive resemblance to Otogakure's flak jacket-minus the steel buttons on the shoulders and the stomach pouch. Also the belt clasps are on the right side rather than the left.
Otogakure: Otogakure flak jackets are black. They seem to be fastened by clasps on the left side and have a pouch in the front. Other than that, they bear a resemblance to those worn by Konohagakure.
Takigakure: In the anime, Takigakure flak jackets are blue. They are otherwise identical to Sunagakure's flak jackets, minus the scroll pouches, neck guard, metal shoulder clasps, and shoulder padding. Additionally, some Taki shinobi wear metal shoulder-guards attached to their flak jackets while others opt for two pockets near the stomach like the the ones worn by the Taki Kunoichi.
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 resembles the older model of the Kirigakure flak jacket and comes with 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, it left the upper sides of the torso exposed while the lower half was only protected by thin padding.
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 and resemble the Konoha ANBU over-vest from the Second Shinobi World War (minus the shoulder guards). They 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 fishnet 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 and resemble the flak jackets used by the Land of the Sky (minus the lower flat used to protect the groin).
Benisu Island: In the anime, the flak jackets worn by the shinobi of Benisu Island are beige — roughly the same colour as Suna flak jackets. 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 triangular symbol embedded in the back (like the Uzushiogakure symbol on Konoha's flak jackets.) 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 resemble Konohagakure's flak jackets; minus the Uzushiogakure crest, steel buttons, and the scroll pouches. Instead, they opt for three built-in equipment pouches on the lower back like Iwagakure's.
Fūma clan: The flak jackets worn by these shinobi are dark blue. They resemble the ones worn by Sunagakure but lack scroll pouches and a neck guard. They are also come with black bands on the side and are fastened over the top with the same type of bands with metal clasps along with a zipper. They also come with iron gauntlets and a fūma shuriken holster on the back.
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 flap beneath to protect the groin. They also bear their village symbol integrated onto both of the shoulders which is reinforced with additional padding.