Polyamory breaks down into two roots: poly, meaning many and amor, meaning love. To sum it up, polyamory is about believing that love does not have to be limited to one partner anymore than it would have to be limited to one child or one friend.
If you hang out in the poly community (online or in person) any amount of time you will no doubt hear that "there are as many ways to be poly as there are people who are poly." While my pithy saying above sums up the concept pretty well, there is a nigh endless number of ways to put it into practice.
There are swingers who have completely open sexual mores and don't feel that sex should be limited to people you have an emotional attachment to. These people are not necessarily polyamorous since love is not a critical component of their sexual encounters (being in an open relationship and being poly are not always the same thing). However, some of them, like my husband, are open to both new sexual partners as well as new emotional partners, and these people fall soundly into the poly camp.
There are also people like my boyfriend who define themselves as polyfidelitous. These people still believe strongly in maintaining sexual fidelity to the partners they are in a relationship with. The fact that they may be in a relationship with multiple partners does not, in their minds, mean that sex is any less of significant emotional investment that should only be shared with in the confines of love and commitment.
And then there is the whole spectrum of in between. People who believe in having primary partners, secondary partners, and tertiary partners. People who have 2 husbands, a boyfriend and a handful of lovers. People who only have one committed partner, but believe sex is a beautiful experience to share with beloved friends. People who have a houseful of partners where everyone is equal. People who have a girlfriend in every city they travel to regularly, but don't officially live with any of them. The list goes on. For every combination you can imagine, someone is living it - or wishing they were.
To deal with this diversity of options and specialized community, an entire lexicon of terminology has been developed. Here's a very limited intro glossary:
V - a relationship configuration in which two partners share a third but are not sexually involved with each other (usually modified as MFM or FMF to denote genders).
Triad - a relationship consisting of three partners who are all involved with everyone in the group.
Quad - a relationship involving four people, often two couples who share partners.
NRE (New Relationship Energy) - that happy, giddy, butterflies in your tummy feeling you get when you first fall for someone new (it is important for poly people to make sure their NRE does not cause them to neglect their existing relationships)
Compersion - that happy feeling you get when your partner is finding happiness with someone else (yes, this DOES actually happen. When the GFIL and Lorien announced that they were officially boyfriend and girlfriend Karma and I squeed like teenagers and did happy dances).
Cheating - breaking the rules; betraying a trust (Sleeping with someone other than your spouse isn't cheating if she's okay with it. Sleeping with your girlfriend's sister when she's told you not to, however, still would be. Polyamory requires trust and communication; it's not an open license to do whatever you want).
For a (perhaps unnecessarily) complete list, check out http://www.xeromag.com/