The latin word 'Librum' means a pound, and the abbreviations derive from this.
The L from librum was first of all crossed through, to distinguish it from an ordinary L. This eventually transformed into the modern £ sign.
The old system of money using Pounds, Shilling?s and Pence? was changed in the UK in 1971 to a decimal system.
It is also a measurement of weight, used in the [Imperial system]? (often referred to as the English system in the US). The letters 'Lb' are used in the Uk and in the US they use '#' to denote the weight. In the UK to distinguish the weight from the currency the currency is often referred to as pound sterling, or sometimes just sterling.