Minecraft 101: Wither
The Wither is a three-headed mob with skeletal kind of appearance. It is the second boss mob introduced in Minecraft, the other being the Ender Dragon.
HOW TO SPAWN A WITHER
By placing soul sand in a T shape, and putting 3 Wither Skeleton Skulls on top of each three upper blocks, the Wither can be created. The last block placed must be one of the three Wither skeleton skulls; it will not spawn if the final component placed is soul sand.
WEAPONS NEEDED TO FIGHT THE WITHER
Because the Wither does a great deal of damage, the best equipment for combat is a diamond sword (Preferably enchanted with Smite. The best sword for the fight is one enchanted with Smite V and Knockback II) Instant Health II Splash Potions, Strength II Potions, one or two enchanted golden apples, a Bow (Preferably enchanted with Power, the best bow for the fight would be Power IV or V, Punch II, and Infinity I) and Diamond Armor (Protection IV protects from the withering effects. The 'Thorns' enchantment may also be an advantage, but it is expensive and not as important as proper protection). Make sure to bring about a stack or two of arrows if you don't have the Infinity enchantment. Fire enchants do not work on the Wither. Smite is better than Sharpness, but a high level Sharpness should be good enough.
For more info about the Wither go to this link: http://www.minecraftwiki.net/wiki/Wither
-Raphael