No questions are stupid as far as IGF is concerned.
But I'll answer them. There is "no" cost to IGF, it is 100% free. And yes, we only play online. Each week there is a new event (or war inside our clan). For example, this weeks event is called "4th Of July Fireworks" and you are limited to certain classes that you can play as. When a match is finished, a screenshot is taken to keep track of the scores, and then those scores are entered into a ranking program for each event. There's also awards that you can earn for playing a certain number of matches in one event and for various other things.