Now that I'm post-primary, it seems worth reposting my personal definitions of polyamory, swinging and nonmonogamy, minus the specifics of how Dodi and I structured our now-defunct relationship. As always, these are generalities based on my experience rather than something textbook-y or canonical. Feel free to call bullshit.
Polyamory is making emotional and sexual commitments to multiple partners. This sometimes produces closed groups, where the members are intimate with one another, but not with outsiders.
Swinging is generally no-strings sex with strangers or acquaintances outside your primary relationship. Sometimes they're one-offs, sometimes they're ir/regular dates. But the interactions tend to be sex-centric rather than about intimacy.
Nonmonogamy -again, defined by my experience- falls somewhere between the two. In my case, I usually have a handful of partners. I consider them all friends to one degree or another, and our sexual interactions wax and wane. At times, I may be intimate with two or three people on a regular basis, at times one, at times none. The commonality is that I do my best to be honest and respectful toward each of them, while still maintaining my independence. They are my friends and my lovers, but they are not my partners.