Seeing as how you're in the SoCal area, I suggest heading over to SoCal Airsoft.
Not that you won't find your answers here, because you probably will, but a more localized scene will be better taylored to your specific questions. Rules on FPS and other things vary greatly from place to place.
As to the basic structure, it's simple. Games have objectives. You shoot each other. It's like what paintball used to be before the speedball craze soaked in, but without the paint, and with realistic guns and gear.
Question is not stupid at all, especially if you try to extend it a bit. I would like to put it that way: "how do you play airsoft in your country/state? Is it more like sport - competing and so on, or - like in Canada and Poland - just a fun with no losers and winners".
Frankly speaking, I couldn't imagine Airsoft being a "sport", it just... doesn't fit. But that's just my opinion.