"Poly" is short for polyamory or polyfidelity.
Essentially, the term means "many loves." A poly person is one who engages in a generally non-monogamous relationship style. These people feel it's natural for them, or the relationship they are in, to romantically involve more than one person. (This is opposed to serial monogamy, where people love as many as they want, as long as they get rid of the last lover before starting in on a new one).
This may mean that all partners are involved romantically/sexually with one another, or that one or more partners have other people with whom they are involved romantically or as close friends who share sensual experiences.
Some people refer to their arrangement as an "open" relationship.
You can learn more about the various forms of poly from the following resources:
•The Ethical Slut: A Guide to Infinite Sexual Possibilities. Dossie Easton and Catherine A. Listz. (1998 Greenery Press, San Francisco)--a common sense guidebook for the sexual adventure.
•Loving More Magazine (http://www.lovemore.com) is a quarterly publication dedicated to the poly lifestyle.
•Anything That Moves is a bisexual magazine has run several articles about polyamory.
•Poly folk in the Portland/Vancouver area might be interested in an e-groups public mailing list. Subscribe by going to http://www.egroups.com/group/PDX-POLY.
•The alt.polyamory Usenet newsgroup FAQ can be found at http://www.faqs.org/faqs/polyamory.
A search with any Internet search engine using the keyword "polyamory" will provide a list of online resources on the subject.