Okay, let’s break it down 
First of all, no, there are no set rules or guides, but the way terms are generally used is a good place to start.
Let’s begin with one you didn’t use, but which sheds some light when used in contrast: polygamous. One spouse marries more than one partner. When it’s practised, it’s generally a cultural institution, and usually it’s either one man man, many women, or the other way round. The Mormons had a “one man, many women” thing going on (do they still?) to raise their numbers.
Notice how I didn’t mention “love” in the last paragraph? Polygamy serves a societal purpose or is a societal institution. Feelings are often secondary. Polyamory is about feelings; people who love or want to love more than one person at the same time romantically enter into polyamorous relationships. Everyone involved knows there are other partners involved, and the relationship networks can get pretty complex (I won’t get into that here, but I’ll be happy to explain polycules, metamours and the like later). Many see “polyamorous” as a kind of sexual orientation modifyer; people can be straight/bi/gay/pan/etc… and poly on top. I think it’s clearest when people describe themselves as “solo poly”, meaning they have no current partner, but if they did, they’d want others to join, too, if the right ones came along.
Now, notice how I didn’t mention sex in the last paragraph? An orgy is group sex between n consenting adults, with n usually being 4 or greater. Why they do it is up to them, there’s all kinds of reasons, many having nothing to do with romantic love (I don’t judge, just pointing it out). There’s also varieties, I would say, depending on whether the focus is on one person (a gang bang), it’s about everyone having fun with everyone else (a classic orgy) and so forth. You could have an orgy with your polyamorous partners, but that’s not generally what happens. It’s more often than not a bunch of couples “linked” by one person. So Joe and Sally are dating and having sex with each other. Mike and Billy are dating and having sex, too. Sally and Mike are also dating and having sex, all parties involved know about that and are cool with it, but they don’t get together for four-person sex where everyone fucks everyone else. Not because it’s forbidden, it’s just that the attraction is usually not there; in the example, Joe might also not care for men in that way.
You’ll sometimes have a group that’s practising polyfidelity: Anyone in the group can have sex with everyone else who’s part of the group, but no one outside of it.
It all comes down to what everyone involved wants, communicates, and gets consent for. Polyamory is a lot of work and discussions!