Armor is effectively protection, and a health boost.
The Bulletproof Vest has 1000 HP, and gives no additional resistance.
The Armor Suit has much more health, (1500 HP,) and used to make you invulnerable to small arms, such as M16, and bestow a blast and frag resistance, while still not able to withstand a direct RL hit. However, after complaints, it was nerfed to 80%, and then finally down to a flat increase.
The Anti Explode Suit gives great resistance to explosives, can withstand multiple RL hits, and, like the Armor Suit, used to have immunity to Small Arms. It also gives more health.
Vehicle Armor adds extra resistance to damage and more health.
Tanks: You have a choice of Extra Front Armor, which stacks with current front armor resistance, or Extra Side Armor, for better side protection.
ATV: Extra Health.