The following is a very basic discussion of the words that we, as jewelry makers and jewelry consumers, use everyday. Every field of endeavor has its own vocabulary, and jewelry is no exception but, because so much money is in play, it is especially important for you to know the meaning of the common terms that are used in the trade to specify the purity (and hence, the value) of the various metals used.
There are three precious metals which are used in the vast majority of fine jewelry: silver, gold and platinum. Although many other materials are used as well (e.g. copper, brass, bronze, cobalt, titanium, palladium etc.), these three are the mainstays and the most valuable. Each begins as a pure metallic element, and each is then combined with other metals to achieve desired physical characteristics (such as hardness, color, durability).
Pure silver is rarely used as the main element in a piece of jewelry. It is very soft and easily damaged. Typically it is mixed (alloyed) with other metals such as copper to improve its physical properties (e.g. hardness, tarnish resistance etc.). The most common alloy of silver is Sterling, an international standard for centuries. While the word 'silver' can mean anything from pure (100%) silver, referred to as fine' silver, to a silver-colored metal with very little or none of the element silver, the term Sterling has a specific (legal) definition: 92.5% is silver by weight, the other 7.5% being various other metals (also selected for what they lend to the end-product). Sterling jewelry will be stamped 'Sterling', 'Sterling Silver' or '925'.
The purity of gold is expressed as a number up to 24 plus the word 'karat'. Pure gold is 24 karat (or 24K) which is, like pure silver, too soft for practical wearability. The percentage of the element 'gold' in a piece of jewelry is the ratio of actual gold in the piece to what would be pure gold. For example, 18K gold is 18 divided by 24 or 18 parts in 24 or 75% gold; 14K is 14/24 or 58.3% gold, and so on. The higher the karat, the greater the value, all other things being equal, with any piece made from 14K or higher being considered 'fine' jewelry; you may come across items stamped 9, 10 and 12K, but these pieces are more in the family of costume jewelry. Depending on karatage, gold will be stamped 14K or .583; 18K or 750, 22K and so on.
Although the price of platinum per ounce today is below that of gold, platinum is much denser/heavier than gold for any given volume; in other words, a piece of jewelry made of platinum will weigh much more than the same piece made of gold...and platinum is much more difficult to work with, so it takes more time/labor to make something. The purity of platinum is expressed as a percentage of weight: 900plat is 90% platinum by weight; 950plat (the international standard) is 95% platinum by weight. Alloying metals are typically iridium or ruthenium.
Finally, Federal law requires anything sold in the United States as silver, gold or platinum, whether made here or imported, must not only have a karat stamp specifying the percent (by weight) of the precious metal, but also a trademark (manufacturer's mark). Essentially, the trademark is a registered, proprietary symbol that identifies the maker, who has legal liability for the accuracy of the karat stamp. Although the industry today as a whole is much more trustworthy than it was a century ago, 'caveat emptor' is still good advice.