White gold is an alloy of gold and white metals such as silver and palladium. White gold can be 18kt, 14kt, 9kt or any karat. For example, 18kt yellow gold is made by mixing 75% gold (750 parts per thousand) with 25% (250 parts per thousand) other metals such as copper and zinc. 18kt white gold is made by mixing 75% gold with 25% other metals such as silver and palladium. So the amount of gold is the same but the alloy is different.
When white gold rings are new they are coated with another white metal called Rhodium. Rhodium is a metal very similar to platinum and Rhodium shares many of the properties of platinum including its white color.

The Rhodium plating is used to make the white gold look more white. The natural color of white gold is actually a grey color. The Rhodium is very white and very hard, but it does wear away eventually. To keep a white gold ring looking its best it should be re-rhodium plated approximately each 12 months.