Dave Winer says A Full Peer is a computer that can be both a client and a server and is operating 24 hours a day 7 days a week. It can call other computers and can be called by other computers. A non-full-peer can't be a server - other computers can't call it. Usually it's a FireWall or a NAT that limits a computer in this way. Sometimes it's the ISP imposing the limit, or a corporate network. It's all the same - if you can receive incoming requests and process them your computer is a peer.
Edited: | Tweet this!