This is how Wikipedia describes two-party systems, but don't take the things Wikipedia says for granted.

http://en.wikipedia.org/w/index.php?title=Two-party_system&oldid=52910492

This is how v-a.com desribes it.

http://www.v-a.com/communication/two-party_system.html

This is a discussion on Google Answers about the U.S. party system.

http://answers.google.com/answers/threadview?id=122256