(Buy Menu Description) Body protection against projectiles and explosives.
The Kevlar Vest is an essential piece of equipment that should be bought at every opportunity. Not only does Kevlar protect against bullet-punch, or the slowdown that occurs when you are hit, it absorbs a significant percentage of damage caused by explosions and low velocity projectiles and can easily be the difference between life and death in a firefight. There are some weapons, like the AWP, that can kill in one hit despite having a vest. Kevlar by itself does not protect against headshots.
When to Buy
Armor is a crucial component to any player's arsenal in Counter-Strike. Without kevlar, being shot by most weapons causes the player's screen shakes violently ("aim punch"). This makes hitting enemies virtually impossible. Moreover, SMGs like the PP-Bizon and MP7 are capable of acing an unarmored team in a single clip. For these reasons, buying armor on a round you plan to win is more or less essential, except occasionally for AWPers. If teams have very little money, they may opt for an eco round (cheap weapons and no armor) so that everyone can buy armor, rifle, and grenades the next round. Teams usually do not buy armor for eco rounds.
In some cases (after losing pistol round, for example), teams may opt for a pistol/armor or force buy. These are rounds which teams plan to win despite lacking the optimal armor+rifle+grenade set. In these cases, most players will opt for either kevlar or kevlar + helmet.
Kevlar + Helmet is always better than just kevlar. It is also $350 more expensive. As CTs, helmets make little difference as long as the Ts have AKs and AWPs. However, if Ts eco or buy Galils, a helmet can still be a game-changer. Ts are usually well-advised to get helmets, as the M4A4 and M4A1-S require an extra shot to kill a helmeted opponent.
Outside of Competitive Matchmaking, most game modes will automatically give players both armor and helmet.