On most servers, you can play both timed games and untimed games. In a timed game, if you exceed your time limit you lose. Times limits are stated in the form "20/5", "60/10", etc. What this means differs on different servers. On most servers "m/n" means that you each start with m minutes, and when this runs out you are in byoyomi of n minutes for 25 moves. On Yahoo it means that you each start with m minutes, and for each move that you make, you receive an extra n seconds.
The better Go servers support ratings of players. This makes it possible for you to select an opponent of your own strength, or to set an appropriate handicap.
The rating systems differ from the EGF ratings system and vary from server to server. When you start playing on a server, you will be "NR" (not rated), or you may assign yourself a rating. After a few games, the server will assign you an official rating.
One should try to be polite at all times. Bear in mind that the people you encounter will not necessarily have English as their native language, if they speak it at all.
One should be willing to resign a lost game. After the game has come to its logical conclusion, one should not attempt invasions that cannot possibly work in an attempt to induce a loss on time.
Occasionally, a player will disconnect during a game. Although this can be done maliciously, it is far more commonly caused by a simple computer failure of some kind. Wait a few minutes - your opponent will probably reconnect and you can continue your game.