The main reason Kazaa and its FastTrack brothers can stay decentralized and yet resist being as dog slow as the ancient Gnutella is that users' upload lists and subsequent queries are sent to designated local super-peers, called Supernodes.
Any Kazaa client can choose to become a Supernode within the options dialog. A checkbox labeled "Do not function as a supernode" is checked by default, and it's a simple matter of unchecking this and reconnecting to the network to find yourself a vital part of the Kazaa experience. Don't you feel all warm and fuzzy now.
As of version 2, Kazaa adopted a participation level system to encourage proper and righteous filesharing. This system involves a dynamic number which supposedly measures your share-fu - that is, how much you give in comparison to how much you take. The upshot of our magic number is seen when you go to download a file: participation level determines your priority on other users' upload queues. More recent developments (namely, the hacking of Kazaa Lite into a userbase-developed K++) have proven capable of circumventing the issue by freezing client-side participation levels for both uploads and downloads.
Kazaa Lite is an excellent alternative to Kazaa, dropping the spyware as well as some bitrate limits on mp3s, and it comes with optional little tool-apps to make downloads speedier and more efficient, as opposed to the "optional" spyware and adware of the original Kazaa. But don't take my word for it!
A vast, steaming heap of information, relevant files, and FAQs can be found at http://www.k-lite.tk