Karat and carat refer to two different measurements. Karat is the purity of the metal, and carat is the weight of the gemstones.
Real gold is generally marked with a karat description, which break down to 10K, 14K, 18K and 24K. All are measurements of how pure the gold is. A listing of 24K means that the item is pure gold. The other karats have other metals mixed in with the gold. The amount of other alloys is determined by the description. For example, 10K has more metal than 14K or 18K.
Carat is the weight of the stone in the piece of jewelry. A carat is 1/5 of a gram. The weight is put into increments of 100. A 1/4 carat is a 0.25ct weight stone.