What are the percentages for on Super Smash Bros. Brawl?

I play that game a lot and love it and I'm pretty good but I don't understand the percentages. It seems like you can just keep hitting someone and their percent can reach a gazillion and nothing happens? Please explain! Also, how do you do the special moves for Zelda/Sheik? I think I have them all I just want to make sure. Thanks!