An unarmed attack provokes an attack of opportunity.
The creature you target with your unarmed attack gets an attack of opportunity against you, provided that it is armed (that is, holding a weapon, has a natural weapon, or has the Improved Unarmed Strike feat). Other foes that threaten you don't get attacks of opportunity against you.
Technically, the creature you attack gets an attack of opportunity against you even if you're not in the area it threatens (or when it's too small to threaten the area around it) because you're reaching into its space to make your unarmed attack. For example, if you're size Medium and you use your 5-foot reach to make an unarmed attack against a Tiny creature (reach 0 feet) that has a weapon (natural or otherwise), you provoke an attack of opportunity. If you've ever tried to pick up an angry cat, you'll have no difficulty appreciating what the rules are trying to simulate here.
