Any permanent can be tapped, although planeswalkers and enchantments are rarely tapped. "Tappedness" is a property of all permanents. Doppelgreener's answer explains the situation well, but here are the supporting rules:
From the comprehensive rules:
110.6. A permanent’s status is its physical state. There are four status categories, each of which has two possible values: tapped/untapped, flipped/unflipped, face up/face down, and phased in/phased out. Each permanent always has one of these values for each of these categories.
502.2. Second, the active player determines which permanents he or she controls will untap. Then he or she untaps them all simultaneously. This turn-based action doesn’t use the stack. Normally, all of a player’s permanents untap, but effects can keep one or more of a player’s permanents from untapping.