The word karat (in the USA and Canada) or carat (in the rest of the English speaking world) refers to the purity of gold, or of gold coins.
(Confusingly, the word carat is also used to refer to the wright of gemstones, one carat being 0.2g, but this is a different system to gold carat purity.)
24 karat gold is 99.99% pure (as pure as you can get in practical terms). Some modern bullion coins of this purity include the American Buffalo, the Australian Nuggets/Kangaroos, the Canadian Maples and the Chinese Pandas. These coins are ‘soft’ and shouldn’t be handled more than necessary.
The Australian Nugget or Kangaroo – 24k or .9999 pure gold
22 Karat gold is 91.6% pure. The gold is alloyed with silver and/or copper. This makes the coin more hard wearing and resistant to scratches and damage, while still retaining the ‘goldness’ and untarnishability of the coin. Some examples of 22k bullion coins are the South African Krugerrand, the American Eagles, and the British Britannias.
The Krugerrand – 22k or 0.916 gold
Many historical gold coins minted for circulation are 21.5 karat. This equates to 90% purity, or 0.9000 actual gold weight. For example, the Coronet Head gold Eagles and half eagles are 90% gold, and 19th and 20th century 20 franc coins from France and Switzerland or Austrian 10 coronas are also 21.5k.
The Austrian 10 corona coin (restrike) – 21.5k or 0.900 gold
As far as the desirability of a particular gold bullion coin goes, all ‘one ounce’ bullion coins have an actual ounce of gold – coins like the American Gold Eagle and the Krugerrand weigh more than one ounce because of the alloying. Many people worry that these coins have ‘less’ gold than 24k or 99.99% pure gold coins, but that is not true.
What about coins with lower carat values? There are coins made of 75% gold (18 carat) or even lower, being made now for collectors or as souvenirs of special events or anniversaries. (This is a common purity in gold jewelry.) As they are not standard purities for coins, you may have trouble reselling coins like these to dealers or collectors who are not familiar with them.
Further reading:
Wikipedia has a fascinating article explaining where the word ‘karat’ or ‘carat’ came from.