The pound sterling is the monetary unit of Great Britain. Some sources consider that this name appeared in the XII century and meant literally — a pound of pure silver. Ancient British silver coin was called sterling. 240 coins weighed 1 tower pound - about 350 grams. This measure have been used for the large buyers, with this they checked the full-fledged coins.
Now in the English language people use the word «pound», in the official documents - the full form «the pound sterling», in the stock market - the name «sterling».