Some creatures have abilities that aren't fully explained on the cards. Most of these have reminder text that gives you a brief description of the ability's effect. Here are full explanations of the most common creature abilities:

Defender

Creatures with defender can't attack. They make excellent blockers and usually have high toughness.

Fear

A creature with fear can't be blocked except by artifact creatures and/or black creatures. Fear only matters when the creature with the ability is attacking.