Pure gold is a warm yellow colour.  It is easy for jewellers to shape and fashion, but it scratches easily and is not very durable in jewellery.  For centuries jewellers have been mixing (alloying) other metals with gold to make it stronger and more durable.  The most commonly used alloying metals are silver, copper, zinc, and palladium.

The amount of gold used in the alloy determines what is called the metal's "carat".  The word carat is abbreviated in the jewellery trade to ct. (or K in America).  There are set levels of gold standards internationally - 9ct, 14ct, 18ct, and 24ct.  They consist of the following:

24ct contains 100% pure gold. 
18ct contains 75% pure gold.  
14ct contains 58.3% pure gold.
9ct contains 37.5% pure gold.

These are the gold standards generally used in Ireland, internationally there are other standards such as 10ct, 15ct, and 22ct.