A community interacts with one another. An online community is exactly like that. When you first start out playing games online you will notice people using a wide range of 'nicks'. What ever name you choose stick with it. Other players will see your name regularly on the game servers and after a while you'll be chatting to them, and build up a friendship with the other players.
Every game has a number of MODs.
These are modifications made to the basic games you get from installing the cd.