Def: the practice, desire, or acceptance of having more than one intimate relationship at a time with the knowledge and consent of everyone involved [wiki]. Some see it as the ultimate fantasy. For others, it's damnation. The way I see it, it's a part of life and of human nature.
Growing up in a predominately Christian society, the only way I saw how relationships work was: be abstinent. get married. be with one person forever. Like many (but, not all to be fair) Christian beliefs, this really limits a person's experiences, imagination, and freedom to grow. And, it's because of these narrow minded beliefs that I became sexually frustrated as a teen. After being out on my own, I saw that it didn't have to be that way.
Even in this day and age, where everything is out in the open, polyamory is a concept that some find difficult to accept. Especially in a western culture, there are still those monogamous influences that prevent many from fully accepting it. But, it is very possible. Think for a moment what it would be like to be in a polyamorous relationship. To be truly open with each person. No secrets, no lies. That is the basis for polyamory.
To me, it's evident that lots of people seek polyamory. How many times have you experienced or heard of someone being cheated on? It's often the basis for many divorces too. With polyamory, you have the option of discussing the possibility of another relationship. It's not cheating because you've both communicated and agreed upon it.
So, why don't people do this? Is it because our society still clings on to the traditional monogamous view? Do people get a thrill out of lying and sneaking around? Personally, I think a bit of both. Also, polyamory isn't for everyone. There may come a point where you're content with only one person and that's perfectly alright. Some people can't handle the jealousy and possessiveness. For multiple partners, each person can bring to the relationship a different aspect. If you feel something is missing in one relationship, then a different person can fulfill that void.
Can it really work? From experience, yes it can. But, you have to be true to the values of polyamory:
fidelity and loyalty
trust, honesty, dignity, and respect
communication and negotiation