Karatage, denoted by a number followed by "k", is the measure of purity or how much gold is in the metal that makes up a piece of jewelry. Karatage is an important factor to consider when buying gold.
Pure Gold or 24K is very soft and malleable, so it is often considered too delicate for everyday wear. In order to make jewelry more resistant, jewelers make alloys, or mixtures of Gold and other metals to increase it's strength. This is where the difference between 18K and 14K arises.
18 karat is 75% pure gold, the other 25% is made up of other metals such as silver, copper, nickel and/or zinc. 14 karat is 58.3% gold, meaning that 42.7% is made up of those less precious metals.
Pure gold does not tarnish, though it is sensitive to scratching. Gold alloys like 18K and 14K are less sensitive to scratching. Though 14K gold is the least likely to scratch, it does tarnish, meaning it losses it brightness over time, as the other metals darken because they easily react to oxygen and other elements.
I believe that 18K gold is ideal, as it is resistant enough for everyday wear, has a high intrinsic value due to it's large percentage of pure gold, and maintains its brightness and gorgeous deep color over time.