Gold has the longest and most storied history of all precious metals. It is soft enough to be worked into interesting shapes, and its warm color and scarcity gave it great value in early civilizations. It has been the foundation of many monetary systems, and remains important to our economy even today.
As jewelry, it was gold's softness and natural beauty that made it appealing, in addition to the fact that it doesn't corrode or tarnish. It is so soft, in fact, that pure gold is rarely used in jewelry. It is mixed with another metal, usually copper or silver, to make a stronger gold alloy, or mixture of metals.
The quantity of gold in a given alloy is expressed in karats (abbreviated as K or KT). Pure gold is 24K; 18K gold is 75% gold and 25% other metals. In other words, each karat is equal to roughly 4.17% of the total of the alloy. As the karat weight drops, the metal becomes more durable but less yellow. Sometimes gold that is a lower karat weight will be plated in high-karat gold to enhance the color. This is perfectly acceptable as long as you pay a fair price.
Also keep in mind that gold plating will wear off with time and your jewelry may need to be re-plated. When buying gold jewelry, look for a stamp with a karat mark, the manufacturer's registered trademark and the country of origin.