kar·at | ˈker-ət
A unit of fineness for gold, equal to 1 part of pure gold in an alloy
Pure gold is designated 24K and is much too soft to be used in jewelry, except as decoration.
If a jewelry piece is 14K gold--the standard fineness in the United States--it means that it is 14 parts gold and 10 parts base metal.
Rings (particularly men's rings) are often 10K gold, due to the alloy's higher durability. The purest form of gold used in jewelry is 18K. It was often used in ancient cultures as a symbol of wealth and royalty.
Note: "karat" to be confused with "carat".