Skeletons

Skeletons are ranged hostile mobs that shoot arrows at the Player.

Appearance
Skeletons have gray bone bodies and the same basic heads as most other hostile mobs. They always carry bows in their right hand, which can rarely be dropped upon their death; in those cases, the bow also has a chance of being enchanted. Skeletons can also spawn with the ability to pick up armor, and in those cases, they can spawn already wearing armor, which has a chance of being dropped.

Uses
Skeletons are the most readily available source of bones, and thereby bone meal, a useful resource. They can also droparrows. Each skeleton will drop 0-2 bones and/or 0-2 arrows upon death. Although skeletons' fired arrows can often be seen stuck in blocks, those cannot be picked up by players. Because of its valuable drops, dungeons containing skeleton spawners are sought for mob grinding purposes.

Behavior
Skeletons spawn in dimly lit areas, and will burn in sunlight. When skeletons are nearby, their presence is indicated by the intermittent soft rattling sound of their bones. Skeletons will chase any player they see within a 16 block radius. They will climbstairs, navigate mazes, and traverse other complex obstacles in order to get within shooting range of the player. When they are within 8 blocks of the player, with a clear line of sight, they will stop chasing and start shooting the player with arrows. If the player moves more than 16 blocks away from the skeleton, it will cease pursuit. Skeletons can attempt to avoid damage from the player by continuously circling around them while shooting at them.

Defensive Measures
Skeletons are vulnerable to daylight (water, shade or helmet protects them), lava, fire, falling, suffocation, and contact with cacti. Surface skeleton populations can be controlled if the player regularly sleeps in a bed at dusk, since skeletons will only spawn if a player is awake at night. Players can keep an ear out for the clicking and xylophone-like sounds of nearby skeletons in caves and use corners and tight spaces to their advantage. If there's no choice, run around in circles and it will repeatedly miss you. Do not try this with other mobs nearby.

Source: http://minecraft.gamepedia.com/Skeleton