Though they may look similar, 14K and 18K gold have a few key differences.
A majority of gold jewellery is not made of 100% gold, and instead is created from an alloy that combines pure gold with a variety of other metals. This is because gold is very soft, meaning that pure gold is easy to scuff, scratch and change shape. Pure gold is also, interestingly enough, an orange color, as opposed to the soft yellow color we normally associate it with.
Gold is measured out of a 24K scale. So, when something is made from 18K gold, that means it is 18 parts pure gold and six parts various metals. This makes the item 75% pure gold.
14K gold is 14 parts pure gold and 10 other parts various metals, thus making it 58,5% pure gold.