It is connecting to peer nodes that it already knows of, unless it is the first time and then it discovers nodes.

Here's the wiki page that addresses your question further.
 - http://en.bitcoin.it/wiki/Network

There are a handful of very excellent overviews:
 - http://omegataupodcast.net/2011/03/59-bitcoin-a-digital-decentralized-currency (1 hour 30 minute podcast)
 - http://linuxoutlaws.com/podcast/215 (2 hour podcast)
 - http://www.youtube.com/watch?v=XQPSwA2Itbs (watch starting at about 0:41 minutes in)