What are Magic The Gathering creatures?

A creature is a permanent that can (usually, but not always) attack or defend against attacks from other creatures. Underneath the picture there will be a short description of what exactly that card is, and it should say whether its a creature or not.

You can also usually identify a creature card by looking at the lower right corner. Two numbers separated by a slash (such as: 6/6 or 12/7) will indicate a creature.