As many others have explained, the karat is a unit of measurement that evaluated how much of an object is made of pure gold. The karat evaluates the object in 24 fractions and identifies them in a binary manner: gold, or not gold. Therefore 24 karat is 24/24, or 100% gold. All lesser values indicate the object is not pure gold, but it tells you nothing about what those other materials are, except that they aren't gold.
So, 14 karat indicates 14/24 parts gold, or about 58%.
The more formal way of evaluating this is 24 x (pure gold mass / total mass).
The word karat (or carat) is thought to have derived from the name of a seed, the mass of which may have provided an early means of measuring and comparing the values of small valuable objects. The karat system is not considered metric and generally overlaps or competes with other methods of measuring fineness, or purity, of a given object.
The purity of gold is measured in Karats, with 24 Karat representing 100% purity. The karats are defined as
K = 24 (Mp/Mt)
where, Mp is the mass of pure metal and Mt is the mass of total alloy.
Hence 24 Karat gold will have 24 parts of pure metal and 0 parts of other metals, making up the alloy.
14 Karat gold will refer to 14 parts of pure gold and remaining 10 parts (=24 parts-14 parts) other metals. In other words, 14 karat gold is
(14/24) x 100 % = 58.3% pure.
In comparison, 24 karat gold is 100% pure.
Note that 100% pure gold is very soft and as such can not be used in jewelry and other applications. Addition of other metals provide these properties and as such, people buy 22 karat or 20 karat or 18 karat jewelry.
Gold is never 100% pure gold. The karat, then, is a measure of how much of it is truly gold, by weight.
This is measured in parts of 24. A 16-karat gold ring refers to the fact that 16/24 parts are actually gold. That leaves 8/24 that is not gold, but other metals.
A 24-karat ring would be almost 100% pure gold. It is close enough to 100% that the difference is too small to detect.
While it seems desirable to have more gold in jewelry and less other metals, it is important to know that gold is very soft and can be damaged easily. Adding in a little of those other metals gives the gold strength and makes it less prone to being scratched, bent or otherwise damaged.
The amount of gold as a percentage of the total is measured in karats. Hence, 100 percent gold is expressed as 24 karat gold, while 14 karat gold indicates that 10 parts are non-gold or alloys and only 14 parts are pure gold. Hence, 14 karat gold represents 14 parts out of 24 pure gold.
Supplementary metals are added to pure gold to increase the hardness or to give a different color. Hence, 14 karat white gold have a white-grey color because only 14 parts represents pure gold and the rest of 10 parts are represented by the additional metals, nickel or palladium, that give the final color.
14 karat gold refers to an alloy gold in which content of pure gold by weight is 14 parts out of 24 parts.
24 carat gold has 24 parts out of 24 parts gold - that is, it is 100% pure gold.
Similarly 12 carat gold will have 50% pure gold and 50% other metals. 18 carat gold will have 75% pure gold and 25% other metals.
Of course we cannot have more than 24 carat gold.
As you probably already know, karat measures the purity of gold alloy.
When measuring a purity of gold alloy, one karat is 1/24 purity by mass. This means that 1 karat would equal approximate 4.2% purity.
As for answering your question, 14 karat would mean that the gold alloy you possess would have approximately 58.5% purity while the 24 karat gold alloy would have 99.9% purity.
Thus, 24 karat would be considered to be pure gold.