Skeletons are common hostile mobs that shoot arrows and drops 0-2 bones, 0-2 arrows, and/or a bow (most likely used, however there is a small chance a bow may be enchanted). They are the primary ranged mob the player encounters. Skeletons are currently only one of two mobs that drop bones, the other being a wither skeleton.
Like zombies, they spawn in darkness and burn in sunlight. Both skeletons and zombies seek shade so that they don't burn in the daylight. When a Skeleton's arrow hits another hostile mob (with the exception of creepers), the other mob will attack the skeleton. Skeletons themselves will also turn on any other hostile mobs that hit them, including other skeletons.