The Wither is one of two boss mobs in Minecraft, (the other being the Enderdragon) and is also the only hostile mob that has to be summoned by the player by building it. Summoning the wither requires you to put four Soul Sand (obtained easily from the nether) in a T-shape and three wither skulls (a rare drop from wither skeletons) on top of the Soul Sand. The wither will then spawn and destroy everything in the surrounding area.
Diamond armor, a diamond sword, and a bow are all highly recommended before you try to face the witherThe Wither will drop a nether star which is used to craft a Beacon. See Beacon page for details.