Counterstrike was freakin' sweet when it first came out. I worked with the guys who made cs_italy, once.
It was a REALLY fun game early on, less so as time progressed I suppose. I haven't played in probably five years. Never played CS:S. If you hear someone referring to the sniper rifle as an AWP, they're an old timer like me, because they used to use an inaccurate name for the real gun and then changed it to be correct.
CS runs about thirty minutes to a map, and each map is entirely self-contained--things that happen in one map don't impact the next in any way. So you can hop onto a server and be on an even level with everyone else pretty quick. Each map is broken up into multiple rounds, which are also mostly self-contained: Only money and weapons carry over, and they're not a HUGE advantage (oh hey, the guy in front of me died, let me grab his gun). Rounds are about 2-5 minutes, and you can't respawn within a round, so be careful and don't just run in stupidly.
Damage is halfway to realistic, moreso than TF2 at least. If you get shot in the head a couple times, you be dead. Hit with a sniper round in the chest, dead. But there's no injuries or bleeding. Still it's realistic enough that sometimes, some guy grabs a P90, takes your team from behind, and manages to hose down everyone at head level--if he's really lucky he just dropped four guys in a second. So don't let him get behind you. Mostly, don't be seen, because if they can see you, they can hit you with a lucky shot. Be somewhere unexpected, and if you can't do that, chuck a flash grenade first.
There's a few types of missions. Like, hostage missions. The CT job is to rescue the hostages (NPCs that follow you around when you get close enough to touch them), or kill all the T's. The T job is to avoid killing the hostages, keep the CTs from rescuing them, and let the ~five-minute timer run out. Or, bomb missions. The Ts have one bomb, and two targets on the map that are marked with crosshairs. Usually, you get some situation like "The CTs spawn on one target, and there's a second target about equal distance away from both teams, plus a back way around between the two spawns, and other corridors". They need to take the bomb to the target, then spend about five seconds to plant it, after which point you have about thirty seconds before it goes off. The CTs can disarm it in about 3-5 seconds, but you can shoot them in the head while they're busy trying, so the CTs want to either hold both targets, or at least sweep through once you've planted it and wipe out any ambushes before trying to disarm. Lots of fun if there's only one guy left on both teams and the CT doesn't know which site the bomb is at.
Anyway, things that help you win the game are generally things that give you money. You get money at the start of each round, you get money from killing an opponent, and from rescuing a hostage, and from being on a team that wins the round; you lose a ton from accidentally shooting a hostage regardless of which team you're on. Even the cheap guns can kill. Usually it takes about two rounds of saving to buy one of the real good assault rifles as well as the body armor you'll want with it, but if you jump an enemy who has that rifle from behind and blow him away, well, it's free. And the body armor can totally save your ass, or not. A helmet won't keep you from getting your legs sawed off by an assault rifle for example.
There's no surrendering or anything.