This game has all kinds of items, but everytime I find one there doesn't seem to be a description. How do I know what all this stuff does?
As a general rule, I don't play Binding of Isaac without having the Wikia Items page open.
There's no real in-game method of knowing what each thing does without trying it for yourself. You will, at times, get a brief, one line description in-game when you pick up an item, but they never really give you enough information.
The Wikia, on the other hand, has a detailed list for every item, power-up, pill, trinket, tarot card, etc., in the game. Easily one of the handiest things to have open when you play.