Flak jackets are a form of protective clothing designed to provide protection from such things as blunt-force trauma or even bladed weapons, shrapnel and other projectiles or weaponry. For the most part, they have superseded the plate armour worn by shinobi during the Era of the Warring States and the First Shinobi World War.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, which can be used to store scrolls and also allow a ninja to access them with only one hand, as seen with Kakashi Hatake's case when he normally does this secretly to extract a kunai.
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). Newer versions feature pockets. 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.
Konoha ANBU (current): Konoha's ANBU flak jacket consists of simple reinforced front and back padding with four straps connecting the two pieces. In addition, they come with steel gauntlets.
Konoha ANBU (previous): Worn during the Second Shinobi World War, this flack jacket consists of an slate grey over-vest worn over Konohagakure's first generation flack jacket. It was connected by straps over the shoulders and came with protruding shoulder guards, an extension to protect the lower 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 clasps on the shoulders.
Iwagakure: Iwagakure flak jackets are 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.
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 a 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.
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, and a neck guard.
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 its only distinctive feature being that it has only one strap to hold it in place.
Kumogakure (previous): This flak jacket 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, and a scroll pouch. It also bore a passive resemblance to Otogakure's flak jacket (minus the steel buttons on the shoulders).
Otogakure: Otogakure flak jackets are black. They seem to have belt-like clasps incorporated into their design and have a pouch in the front. Other than that, they bear a resemblence to those worn by Konohagakure.
Takigakure: In the anime, Takigakure flak jackets are sky 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 pockets like the the ones worn by the Taki Kunoichi.
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. They have no sleeves and appear to be completely plain. Other than that, they come with a throat guard and shin guards, are attached by straps over the collarbone and extend slightly downward to protect the lower stomach.
Prajñā Group: Flak jacket 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 a back pouch similar to Iwagakure's.
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, 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.
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 lower abdomen. They also bear their village symbol integrated onto both of the shoulders which is reinforced with additional padding.
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).