Infinite Keys are keys that will let you open one of the Infinite Crates. Example, the Infinite Wool Crate. When you open it, you have a chance to get one of the 16 different colored wools Minecraft has to offer, but an infinite amount of it. You have a lower chance to get the cycler (in every infinite crate), which is an item that can cycle through all the different colors of said material.
Misc keys are keys that aren't part of anything major. Some examples being; vote key, spawner crate key, and the coinflip key. These are labeled as misc because they're not worth much. The vote key opens kind of a starter crate, the vote crate. The spawner crate just contains spawners. No infinite spawners, just a one time place spawner. Coinflip crate only contains the nugget of lose or $50,000.