1- the tracker keep the files hash in store, in file or database format.
2- users that download these files hash will be in continuous contact with your server to keep the tracker up-to-date for the statistics... that's the big problem with this system. some new systems never update their stats if you can't finish a download, so there is less contacts with the tracker... but you have always a contact with the server, so there is a lot of bandwidth if you host hundreds of files...

i actually host myself 3 trackersfor 3 softwares companies... they are very server intensive, but they pay for it... and they are only distributing their own softwares. ... yes, the more we see companies distribute their games or softwares with torrentbit format because it's less server intense than a ftp...

but you have to know what is your market before starting... if you have games or big images, you will havea lot of leechers, not much seeders...