The words Carat and Karat are both correct but are commonly misused in references about jewelry.
Carat is a unit of measurement for the weight of a gemstone. A carat equals 1/200 of a gram. The word carat comes from the carob tree, whose seeds were used as balance on a scale because the seeds were of uniform weight from region to region.
Karat is the term used for the fineness of gold. The highest is 24 karat, which is pure gold (.999 pure). If an item says it’s 18K, that means it is 18 parts gold and 6 parts other alloys, used for coloring and strengthening the metal (to equal 24K, or 75% gold or 750). The commonly recognized 14K gold mark is 14 parts gold and 10 parts other alloys to color and strengthen the metal (to equal 24K, or 58.5% gold or 585).
These quality marks can usually be found on the inside of a ring or other jewelry item.