What is a class ID? I need to know.

You're asking it because you've seen it in the Sprite DB, right?

NSMB associates sprite ID's to class ID's.

A "class" is a piece of program code that does something, explained very simplified. So the Class ID's for the sprites tell us what class is associated with each sprite. For example, class ID 325 points to a code that does absolutely nothing. So we can know for sure that if a sprite has Class ID 325, it will do nothing, so there's no need to test it.

Don't worry about it too much, it's not important unless you're doing advanced ASM hacking.