There are many things. For starters its a first person shooter. There are 7 classes: Scout Soldier Pyro Heavy Demoman Medic Spy And two teams, RED and BLU. There are many game modes, for example: Attack/Defence Capture the flag Payload Control points And some others. You find weapons in the game randomly, you can buy/sell/trade items too. @MappyTurtle can explain better than I can. Its a fun and addictive game!
So pretty much, you search people up in Community. Then if someone sends you an invite, there should be a green envelope near this thing saying Cryomancer0's account. Click the envelope to see who sent the invite. I'll send you one right now.
it shows you if they're online or offline. you can also play the game they're in too. It's pretty much just a way to be more connected with players online. I currently have 7... i'm lonely pls add meh tm30gaming <3