Blizzard has updated their page on socketed items. This page answers all the questions you may have on socketed items.

A socketed item is like any other item in World of Warcraft, except that it can be upgraded by attaching magical gemstones into the item itself. This imbues the item with additional powers similar to an enchantment. The number of sockets and the types of gems used within the item allow for a wide variety of customizations based upon how you wish to improve your character.

Check out the rest of Blizzard's Socketed Items page.