WinMX is similar to Limewire in that it operates on a peer-to-peer basis. i.e. your downloads come directly from another user's PC and not from a central site. While you're downloading music files, other users can access the shared files on your PC. WinMX uses its own network whereas Limewire uses the Gnutella network (which is also used by other software such as BearShare) but the technology is basically the same.
Using P2P networks substantially increases the risks of viruses, trojans and other 'nasties' finding their way on to your PC.
The one-off fee charged by WinMX is for software and 'support'. It has nothing to do with paying copyright owners for their work. WinMX is basically offering a similar service to Download Music Inc. Please read my post about the legal position of such services, here:
http://www.theanswerbank.co.uk/Law/Question353 035.html
Chris