Peer-to-Peer replication builds on the foundation of transactional replication, and can be viewed as a multiple-node transactional replication with each node published to other nodes, and at the same time, subscribed to other nodes.

See:

http://blogs.technet.com/b/patricg/archive/2009/04/17/peer-to-peer-replication-in-sql-server-2008-configure-a-two-node-topology.aspx