Karats are used to describe the purity or fineness of gold. A karat is 1/24th of a portion, so 24-karat gold is pure gold. 14-karat gold is 14 parts gold and 10 parts other metals. 18-karat gold is 18/24 parts gold, or 75% gold.
Pure gold is very soft, so it is usually mixed with other metals to form an alloy that is hard, less expensive, and sometimes has a different color.