It is basically a a group of microorganisms typically found in decaying organic matter. I think more correctly animalcule which covers all the micro animals including Paramecium, Amobeba, Rotifers and even Ichthyophthirius.
They are (with the exception of Ich which you wont grow in a bucket anyway) good food for very small fry like Betta because they a cannot eat anything as big as BBS usually for a wekk or 2.
I make cultures with boiled water in a jar or bowl. Add a handfull of scrunched up lettuce. Within a couple of days there will be tons of life in there from, as Alan says, the atmosphere. Amazing how water and vegetables (or fruit) can make small animals :-)
Some say you can speed up the process by mixing in a small amount of the muck from a filter also.