Gnutella vs. Napster

Napster was a centralized peer-to-peer service.  A centralized peer-to-peer service means that when a user looks for a song, for example, the user hits a centralized server.  The centralized server responds with a list of machines the user may connect to in order to download the song.  The user may then connect to a specific machine and download the song independently of Napster. 

Gnutella is a decentralized peer-to-peer protocol, whereas Napster was a centralized service.  Therefore, there is no single point, like Napster's centralized server, that can be removed to shut Gnutella down.  Furthermore, Gnutella is not owned by anyone; rather it is an open source protocol and there are many freely available client implementations (see http://www.gnutelliums.com/). 

Additional References:

http://www-personal.umich.edu/~drewpull/napster.html

http://visionarymarketing.com/articles/gnutela.html

http://www.wired.com/news/mp3/0,1285,34978,00.html

http://www.openp2p.com/pub/a/p2p/2001/12/14/topologies_one.html