It may seem to some that many of us have a most untidy sampling in our accumulation or "heap" of guns; not really a well defined "collection". Over the years, interests change. For a while, it may be N-frame S&Ws, then Winchester pre-'64 Model 70s, followed by O/U shotguns, etc. That's how many of us wind up with what we have today. Perhaps not quite as disorganized as it appears.
I find this description pretty accurate, as will many, I imagine. Personally, I have come to differentiate between "collecting" and "accumulating" like this:
An accumulation is a personally owned batch, pile, or "heap" of guns, obtained mostly because of an interest in firearms and hunting/shooting, a sense of need or desire for any particular gun, or perhaps due to an opportunity that arises in the form of a good deal or perhaps a whim. The accumulation is generally random, and has no specific theme or "purpose."
A collection, broadly speaking, is an accumulation with a goal, or theme, where additional guns are added specifically to enhance the collection. A collection doesn't necessarily have to be expensive/valuable, as long as it has a theme. I have personally known collectors who greatly enjoyed adding new guns to their collections of Spanish auto pistols, hardware store branded arms, bolt action shotguns, single-shot .410s, and more. Of course, many collectors have more than one collection; e.g., Browning shotguns in one, with another of Winchester lever guns. It is worth noting that most collectors also are accumulators. My S&W collection accounts for roughly half of my own pile of guns. And lest anyone get the wrong impression here, I've never considered collecting to be in any way superior to accumulating.
A collection, then, is defined by the collector's vision. Personally, my goal for my Smith collection has always been to own a "representative" collection; one which has a wide variety of features representing as many different finishes, frame sizes, calibers, barrel lengths, etc. as I could manage. As I imagine you are all aware, S&W is particularly well-suited to this approach, simply because there have been so many variants over the decades.
My thoughts only, of course, and they may only serve to illustrate the fact that for many years, I had a very long daily commute with plenty of time to think about stuff.