From my late teens through my mid-30s, I always classified my sex partners in this order:
Trick: current vocabulary: hook-up: totally random and one-off encounter (two max), occasionally totally anonymous;
Boyfriend: current vocabulary: FuckBud (also called a Regular) : a trick with something so special that intrigues me enough to want to see him more than twice: range of activities, including eating together or catching a movie: limited contact with other friends and family, but still a suitable escort to some minor function if the need arises; familiarity with his life including where he works and his range of interests: average duration would be anywhere from 3-6 months with no expectations of exclusivity whatsoever;
Lover: current vocabulary: Boy Friend or Partner: a shared life revolving around commitments: intense involvement in each other's families: sharing all evening meals (unless out of town), including prep and clean-up: sleeping together in the same bed nightly (unless out of town) until a single home for us could be arranged: expected complete fidelity in the beginning, eventually opening up via discussion and mutual agreement: escorting each other to all functions, whether work or family related and sharing every holiday together; a joint checking account with mutual and equal responsibilities toward all debts accrued either separately or together (except in one case with one lover): the expectation that though the relationship would evolve, we'd never separate.
Over the years, I've had hundreds of fuckbuds; there were many times in my life where I found the arrangement preferable to the intensity of a full-on partnership. Until my late 30s, when I moved away from Boston, easily 70% of my social contacts were fuckbuds who had cooled down to friends (with occasional benefits, sometimes) or ex-lovers.
There was only one exception to this categorization. We met when I was 23 and in an open partnership and wove a strand of lust and passion through each other's lives (despite the break-up of one and the beginning, middle and end of another full-on partnership) until he died in 1990. I freely admitted my love to him, and he to me, but we could not make the move to a full-on partnership as his life was ruled by chaos and drugs. He's not the one who got away, he's the one for whom no rules could be applied.
I wrote about our adventures in a three-part blog post that I posted here a few years back. It's still there if anybody wants to read the whole thing.