No one cares about rugby league. Union is far better and more popular.
Sevens is (funnily enough) played with teams of seven. 14 minute matches, 7 minutes each half. Same basic rules as rugby. For obvious reasons, the pace is a lot faster. Players need to be hugely fit, and whilst rugby union is a game for all shapes and sizes, Sevens you have to be more of an all round athlete, big, strong and QUICK.
The playing field tends to be a lot more level, that's why you get 'smaller' rugby nations like Kenya, Ivory Coast, Russia, Portugal etc doing well and standing toe to toe with the big boys (not always, but often).
Personally, I'm not a big fan of Sevens, but I can appreciate how popular it is, and how it's a far better fit for the Olympics than Union.