*cough* (points to flags)
Polyamorous here. Dating two women at the same time, and yes they both know about it. IMHO, the latter part is very important. Otherwise it's just "I sleep around a lot" and it's potentially dangerous to all parties, physically and emotionally.
There's definitely a spectrum of relationship, from "deeply committed life partner" to (for lack of a better term) "fuck buddy". Some choose to define polyamory as multiple serious relationships, which is tricky but possible. I know of a few people who have done that happily for a decade or more.
For others, it may simply be a network of sexual partners who are aware of each other (and depending on the orientations, may be partners among themselves).
For me personally, it's somewhere in the middle. I have one serious relationship which may someday turn into a marriage. She has a couple of...I believe the best term is "friends with benefits" -- people with whom she has friendships with beyond just sexual relations. Likewise, I have a FWB myself -- we go out to dinner occasionally, go to movies, just hang out and watch dumb TV and make snarky comments, etc. I qualify that as "dating", even though we both acknowledge that there's no long-term goal in mind and no "possession". If she wants to see other guys, that's fine as long as she lets me know, and vice versa.
I think that's perhaps the biggest paradigm shift compared to traditional models of relationships and sex: the traditional idea is tied up with notions of possession and exclusivity (and jealousy). The polyamory model is much more one of free association, consensuality and openness. Because of that, the two really aren't interoperable. You add a monoamorist into a polyamorist network, it's going to cause problems. Which isn't to say that everyone involved has to have multiple partners, they just have to be okay with the general concept.
It's certainly not for everyone, and you seriously have to check your ego at the door otherwise it's way too easy for people to get their feelings hurt.