24 karat (or carat if you like) is 100%, so 14 & 18 are fractions. 9k is more common as a cheaper alternative to 18k outside of the US. People from Asian countries traditionally go for 22k but there is a growing trend away from that towards white metals.
Interestingly, you can add a few percent of copper to gold and the resulting alloy is harder then both copper and gold. Gold is alloyed because pure gold is too soft.