The Superdraft happens every year. It's a chance for all MLS teams to gain young talent from developmental leagues, like the San Jose Frogs. The draft picks you get will depend on the placement you have on the regular season standings. The worse you are, the nicer draft pick you get. It's the NBA draft football style. You can also trade those picks for players around the MLS, gain more picks from other teams for your players, and decide not to pick anymore players.
Oh I forgot. There's four rounds on the Superdraft, and another four for the Supplementary Draft. The Supplementary draft's another chance for the young guns to get a team.
The waiver draft's where all the people waived by their respective club get clumped and drafted like the Superdraft youngsters.
MLS is the most complicated football system in the world isn't it?