Most people are confused about the difference between polyamory, polygamy, swingers, and cheating. Polyamory refers to having more than one, simultaneous, committed relationship where all parties know about the other and consent to the set-up. Polygamy refers to being married to multiple people at the same time. Swinging is when a committed couple engages in casual sex with persons outside of their relationship with the consent, and sometimes inclusion, of both parties. Cheating is sneaking around behind your S/O's back. I can't think of anyone (polyamorous, polygamous, or swinger) that condones that kind of behavior.
How do you know which flavor you're dealing with? Simple.
* If you are dating more than one person at the same time, and both parties know about it and are okay with it - you're in a polyamorous relationship. What we call a V to be exact.