This was a new word for me (as most of them are), as was this, which I thought was cool:

NOTES:
Here's a little chart that explains it all:

monopoly: one seller, many buyers
duopoly: two sellers, many buyers
oligopoly: a few sellers, many buyers

monopsony: one buyer, many sellers
duopsony: two buyers, many sellers
oligopsony: a few buyers, many sellers