The Latin word libra describes a Roman unit of weight similar to a pound, and the abbreviation "lb" for the unit of weight and the sign £ (a crossed-out L) for the currency derived from this. The word "pound" itself comes from the Latin pendere, to weigh.
The term pound is also the used for the unit of currency[?] in several countries, including the United Kingdom, Egypt, and, before January 1, 2002, Ireland: see pound (currency). This unit was originally derived from a pound weight of silver.
In the Imperial system (often referred to as the pound-inch system, or the English system in the U.S.) there are two basic pounds defined, and also an obsolete definition of one variant of the pound:
The pound (avoirdupois), abbreviation 'lb' or sometimes # in the U.S., is the mass unit defined as exactly 0.45359237 kilograms. This is now more unambiguously called the "international pound." This definition has been in effect since 1959 in the United States.  (http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf)
In the United States, the pound has been officially defined as a unit of mass and defined in relation to the kilogram since 1893, but its value in relation to the kilogram was altered slightly in 1894, and again to its current value in 1959 (which only differs from the 1894 definition by approximately one part in 10 million.)
There are 16 ounces in a pound (avoirdupois). The pound is equal to exactly 7000 grains, where a grain is officially defined as exactly 0.06479891 gram. The legal definition of the pound in the United Kingdom and Canada are the same as in the United States, and were unified to their current value in 1960.
In the United Kingdom, the pound was similarly defined as a unit of mass by the Weights and Measures Act of 1878, but having a very slightly different value (in relation to the kilogram) than it does now. The 1878 definition in the U.K. defined pound as a mass, but having a very slightly smaller value (equal to approximately 0.453592338 kg) than it does now. This old value is sometimes called the imperial pound, and this definition and terminology are obsolete unless referring to the slightly-different 1878 definition.
A pound (troy) is a unit of mass in the U.S., Canada, and UK. The troy pound is a unit of mass equalling exactly 0.3732417216 kilograms. There are 12 troy ounces in a troy pound. A troy pound is equal to exactly 5760 grains, making 1 troy pound equal to exactly 144/175 pounds.
They troy pound is used for measurements of precious metals such as gold, silver and platinum. Any weight measurements of precious metals always uses troy pounds and ounces, even though you will find that it is not always explicitly stated that this is the case.
One troy pound = 12 troy ounces = 240 pennyweight.
If neither "avoirdupois" nor "troy" is specified, the international pound (avoirdupois) is meant and is by law the only proper definition in the U.S., U.K., and Canada; the troy pound has been officially abandoned in the UK. The valuation of precious metals on U.S. exchanges is specified as dollars per troy ounce, although the fact that the troy ounce is implied is usually not stated clearly.
Finally, "pound" or its translation is used in many countries that use the metric system as an unofficial term for a half kilogram (500 grams).
The pound, a unit of mass, should not be confused with the pound-force, a unit of force or weight.