Body armor in games acts as additional protection to the health the player would otherwise have. There are a few different types of armor that range from temporary to indefinite protection. They can come in a variety of forms: bulletproof vests, actual armor or just the game telling you that you now have some body armor. They're usually made out of metal or some kind of resistant fabric. Usually it has a separate status bar to your health and in some games body armor regenerates while health does not ( Shield Regeneration
Body armor in an FPS when picked up adds some temporary protection that filters damage as long as the armor has hit points or expands overall health. Usually when the player gets hit, the body armor takes the blow and only a small amount of health is damaged, some other games however, protect health entirely.
In Unreal Tournament
there are two types of armor: normal armor; thigh pads and body armor, and the shield belt. The normal armor provides partial protection and reduces the damage as long as the player has hitpoints. The shield belt gives a sort of health bonus, this means that the shield takes all the damage until it's depleted, when it's depleted the player's health is directly damaged.
Some first person shooters which feature full localized damage
sometimes have their armor divided into separate pieces; helmet, body armor, thigh pads, etc. In this situation different parts of the body are affected differently by damage depending on the presence and/or degredation of the armor i.e. (if you are hit on an unprotected part of the body then your health takes more damage than if it had been protected, of course this also applys to enemies if their armor have this kind of separation and uses the same system.
In RPGs, you can choose what kind of body armor you apply to your character. In these kinds of games, body armor serves mostly to reduce the damage made upon health and some come with special attributes, like water resistance or the ability to alter stats as well whether beneficial or not. A few RPG's feature item degradation
which means that the armor loses its protective power when it's damaged. Unlike in FPS's where armor disappears, in most of the cases the player can repair it or pay a black-smith or an armorer to recover the armor's protective power. Some other RPG's have power armors. Power armor provides its protection so long as it has energy, when the energy is depleted, its energy must be refilled it to regain its protection.
Some games feature body armor as a power up. This means that the player has the benefit of its protection for a few moments before it disappears, this type of armor in most of the cases reduces the damage from projectiles and when the timer runs out the player receives damage as normal.
Some type of armor have features picked from the standard variants and mix and match different aspects of armor found in a variety of games. Some suits can also act as an armor varient but their protection is often weak. Some FPS's use RPG style armor and vice versa, but those cases are rare.