For the use in Jewelry, Gold is strengthened by combining it with other metals or alloys to produce a more durable metal. This combining of other metals with gold not only makes it more durable to use in jewelry, but it also produces different colors of gold. These different colors can be a beautiful change to the regular bright yellow. For example when you mix copper with gold it adds a reddish tint. Here are a few combinations usually seen in the US:

10k Gold - Usually combines silver, copper, zinc & nickel. It is about 41.7% pure gold (10 parts out of the 24) this meats the minimum karatage to be called 'gold' within the USA.

14k Gold - Also usually combines silver, copper, zinc & nickel. It is about 58.3% pure gold (14 parts out of the 24) this is the most popular karat sold in the USA.

18k Gold - Usually combines silver, copper, nickel and palladium (for white gold) 75% pure gold (18 parts out of 24) Most popular in Italy and European countries.

22k Gold - Usually combines silver & copper- 91.6% pure gold (22 parts out of 24)

24k Gold - No additives- 100% pure gold (24 parts out of 24)

The Karat Weight of an item is the amount or % of gold that the piece contains and is very important when you are comparing pieces of jewelry and their prices. 

Gold-filled jewelry- When you hear about Gold-filled jewelry it is a process where a thin layer of gold is bonded onto a non-precious metal. This sort of bonding is done by soldering, brazing and/or welding. For a piece to be called gold filled, the weight of the gold must be at least 1/20 of the total weight of the piece, and the quality must be at least 10K. 

Rolled Gold Plate is similar to gold-filled, but the weight of the gold is less than 1/20 of the metal used. That is the only difference between the two. 

Gold Vermeil is a basic combination of gold and silver, in which a layer of 10-24K gold is bonded or electroplated over a base of sterling silver. 

Gold Electroplate or Gold Plating is when articles plated electro-statically with gold or gold alloys are called gold electroplating. A stamp reading '1/40 10K Gold Plated' means that the gold coating makes up 1/40 of the article's weight, and that 10 karat gold is used. 

Any piece of jewelry you buy should always have the karat stamped with the karatage on the inside or clasp of the item. If your piece of jewelry does not have a karatage stamp, you really don't know the quality of gold you have. 

Super Fine Gold is usually the finest gold jewelry that remain durable and usable in jewelry and that is 18 karat. Jewelry with a higher percentage of gold above 18k may be affected by wear and tear and handling because the higher the karat weight, the purer and softer the gold.