Skeletons are a somewhat uncommon enemy in Spelunky.
Statistics
Health: 1
Damage: 1
Size: Small (1x1)
Corpse?: No
Stunnable?: No
Special: Can pop out from skulls on the ground.
Strategy
Bones can appear two ways. "Fake" bones are those that may pop up as a skeleton
Skeletons can only spawn from skulls on the ground. These skulls will damage you if you run over them or attempt to pick them up. You can tell which skulls will pop up by whipping them - skulls that are part of a skeleton will be destroyed, regular ones will not. They will then proceed to walk toward you. However, a safer way to discern a skeleton from a pile of bones is their "resting" sprite; skeletons always have an extra "bone" just to the right of the skull one pixel longer (and noticeably brighter).
Bone piles which can't be destroyed when shot at (for example with the shotgun) always reveal a skeleton. Be careful; skeletons are invincible when they're in their "rising from a pile of bones" animation, but they can't hurt you, either.
Once spawned, a skeleton will blindly walk forward, turning around when it hits a wall and merrily dropping down all holes.
They are easily dispatched with the whip, or by jumping on them. When destroyed, they drop a skull, but it will usually shatter on the ground.
Trivia
Oddly, a skeleton that falls onto spikes will stain the spikes red with its blood.
Also, if you throw a skull at a skeleton, blood will spurt out (but the skeleton will still die, don't worry).
What the skeleton is doing with blood is anyone's guess.