Sort of like a flee market, some trading, some straight selling/buying. There will be a few vendors there selling/displaying, a raffle, food, etc. It's a really good time if you're into aquariums. (My wife is usually bored)
As far as it being a swap.... for me I usually sell some stuff and then buy at least equal to that amount, so in my mind I just swapped my stuff for some others, even if money had to exchange hands to make it happen...