Pure Gold is 24 karat or 1000/1000, 18 karat gold is divided 750/1000 or 3/4 pure gold and 1/4 other metals. The most common gold alloys are: 18, 14, 9 and 8 karat.
A good example is the difference between 18 karat yellow gold and 18 karat whie gold. Both metals are gold / other metals in a ratio of 750/1000e . But in yellow gold there is more cooper to be found and in white gold more zinc, nickel, silver, platinum and palladium. Because of its composition, white gold is slightly more expensive. (It is usefull to be informed about the composition.)
The purity grade of gold decreases with the decreasing karat: 14 karat gold is an alloy of 14 parts gold and 10 parts another metal or other metals and in 8 karat gold there are only 8 parts gold out of the 24 parts in total.
Difference between carat as a measurement of weight and karat as gold content
The same word is used, althou spelt, differently, for the weight measurement of gemstones (carat, abbrevistaion ct) and to indicate the purity grading of gold alloys (abreviation kt). This purity grading reveals the relation of pure gold / other metals ratio. The content of gold alloys is expressed in karats or thousands.