I'll say it now. Team Fortress is the best online game I've ever played. The graphics are excellent, the game play is fun, and unlike some other first person shooters there is very much a team play element involved. The 'RED' and 'BLU' teams compete against each other on a number of different map types.
There's your standard capture the flag mode wherein the objective is to steal your opponent’s intelligence briefcase while trying to stop them doing the same. Another game mode focuses on capturing control points where the player will either be on the attacking or defending team. The attackers will win if they take all of the enemies control points while the defenders will win as long as they don't allow the enemy to capture all of their control points. Finally in payload maps the attackers have to transport a cart containing a bomb through various checkpoints in an effort to blow up the defending teams base.
If you tried fighting your opponents on your own Rambo style in any of these game modes I'm afraid you'll quickly find yourself back in the respawn area. You'll need to rely on your teammates to help you both attack and defend positions. Your teammates will be one of 9 different classes each of whom have their own abilities. For example a scout is extremely quick and useful for trying to grab the enemies intelligence while avoiding their fire. However they're also the weakest of the classes and can't take much damage before they die. At the opposite end of the scale the Heavy carries a large minigun that can dish out a lot of damage, and with 300 hp he can also endure a lot of pain before going down. However the guy is about as fast as a dead turtle so is probably more suited to defending positions rather than attacking.
Other classes include the Engineer who can build powerful sentry guns to protect key points. His worst enemy is the spy who can disguise themselves as a member of the opposing team and also use a cloaking device to completely disappear for a short period of time. This is useful when trying to take out the sentry guns with sappers.
The Medic is a great example of how team focussed Team Fortress is. As a medic you won't be racking up the kills but you can use your medigun to heal teammates to up to 150% of their total health. If you're healing a teammate while they're killing enemies you'll get points for assists. The Medics ace card is that when he's done enough healing he can unleash an ubercharge that will make the player he's with invincible for a short period of time - another useful tactic for taking out sentries.
The game also has Pyro, Demoman, Sniper and Soldier classes all of which have their own abilities. Basically, no matter how you like to play your online games you'll find a class you'll enjoy playing as. Each class also has their own taunts which add another element of humour to an already highly enjoyable game. The funniest thing I find is when it's someone on the server is playing on their birthday, all of the player models wear party hats and instead of being blown up into a pile of guts, they'll become a nice pile of pressies!
The only issue I have with Team Fortress is the long loading times, though this is probably due to the fact I play on my laptop. Despite this once you're on a server it's entirely possible you'll stay there for hours. If you don't already own Team Fortress I recommend you go out and buy it today! (That said, why don’t you own the Orange Box?! It’s in there too)