There haven't been that many in the episodes. I can recall:
Class D planetoids - rocky, no atmosphere, minor planets (presumably like Pluto)
Class K planets breathable but uninhabitable due to temperature extremes
Class ? What was the Demon planet again? Was it Y?
Class G - I might have dreamed this one but I think it was a gas giant.
I think some non-canon sources have tried to map out the classifications. Has one ever been formally adopted?