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.

How do I place a gem into a socketed item?
If an item has sockets, shift-right-click on it to bring up the item's socketing window. Here, you can simply drag and drop gems from your inventory to the sockets. Once you are satisfied with your selections, click the "Socket Gems" button at the bottom of the window. Be aware that socketing is permanent - the gems are permanently affixed to your socketed item and there is no way to undo your decision.

What can you put into a socketed item?
There are three primary colors of gems that can be placed into a socketed item: red, blue, and yellow. In addition, there are "secondary colored" gems - orange, purple, and green - that match two colors. Players can place any color gem into a socket; however, matching all of an item's sockets with their corresponding colored gems can award additional bonuses. These dormant bonuses appear in grey text below the sockets. There is also a special type of gem called a meta gem, which fits specifically into a meta gem socket.

Can I put a different gem into a socket that already has a gem in it?
Yes. However, any gems that were already socketed will be destroyed in the process - think of it as "overwriting" the old gem with the new one.

Do I have to fill all the sockets in an item at once?
No. You can fill one socket at a time until you find the right gems for other slots, or fill them all at once.

How do I gain a socket bonus?
As stated above, socketed items have dormant bonuses which appear in grey text below the sockets. This bonus will become active when certain criteria are met - usually when all an item's sockets are filled with gems that match the colors of the sockets. If an item has a red socket and two blue sockets, placing a red and two blue gems into the corresponding colored sockets will activate the socket bonus. The socket bonus stacks on top of the stat bonuses the three gems would normally provide.

Note: If a gem matches the socket it is being placed into, the socket will begin to sparkle.