As a comment to what wonko writes, the game is not that realistic, eventhough it is by far the closest thing to reality in FPS games to day. There are some inconsistencies with physics like instant crouch (a running player can crouch and come to a complete halt in exactly one frame) and totally unrealistic auto-fire; you can not tell where the next bullet is going to land by looking at where the previous ones landed but, rather, the bullets are spread in a uniform circle the size of which depends on the weapon, player actions (firing crouched vs. firing while jumping or climbing a ladder which totally ruins accurate firing), and the duration of full-auto firing; short bursts with small pauses in between keep the recoil at bay and makes accurate aiming possible
while full-auto-till-the-clip-is-empty (generally a sure sign of a beginning player) is nothing more than mindless waste of ammunition (and in a tight combat situation very often means certain death, unless the enemy happens to be at near point blank range). Also, grenade explosions don't
take into account any obstacles in the maps but, the damage is done based on a simple linear(!) formula and anyone within the blast radius is damaged.
As a comment to what de-frag writes: that may have been true with the early versions of Counter-strike - the current (at the time of writing) version 1.5 seems very stable. Biggest stability problems have been caused by various mods that the servers use (anti-cheat, statistics etc.) like a cvar detection bug in older versions of CSGuard/HLGuard (a popular anti-cheat mod), which caused player computers to freeze with sounds looping. This bug was fixed in HLGuard v1.00 beta 4.
In CS a player has exactly 20 hit boxes (head, neck, l.shoulder, r.shoulder, l.upper arm, r.upper arm, l.forearm, r.forearm, l.hand, r.hand, upper torso, middle torso, lower torso, pelvis, l.thigh, r.thigh, l.calf, r.calf, l.foot and r.foot) which take various amounts of damage from hits. Hostages have nineteen hit boxes and a particularly large headslot. As a small curiosity even the chickens found on some maps (cs_italy for example) have nine hit boxes and a particularly large headslot (Note: these hit boxes were confirmed by disassembling the player model files). The most vulnerable hit box is the head, which takes roughly 3-4 times the damage a shot in the chest would, followed by pelvis, which takes about 2 times the damage chest would. Pelvis is a difficult hit box to score hits on because it is almost completely hidden under other hit boxes.
Talking about hit boxes, probably one of the funniest but, also the most frustrating bugs that the early versions of Counter-strike had were the extended hit boxes; under certain conditions (crouched, while holding the Dual Beretta Elites) a player had his head hit box extended vertically by about a factor of 4, which made it possible to accidentally score headshots on players you couldn't even see, by shooting in the air above boxes. Currently, there still is a small bug-like feature that (imho) should be corrected: it is much more difficult to score a headshot on a player from behind, because the head hit box is partly hidden behind the upper torso and shoulder hit boxes. Version 1.4 fixed a bug which was still causing the upper torso and shoulder hit boxes to be vertically extended, making it harder to score headshots.