Longitude is a geographic coordinate that measures a point's distance east or west from the Prime Meridian. Longitude is measured in degrees, with the Prime Meridian beginning at 0º, eastern coordinates going up to +180º and western coordinates reaching up to -180º.
The lines that run vertically through a map of the world are the lines of longitude. Each one begins at the North Pole, runs through the equator and ends at the South Pole. The Prime Meridian itself is merely an agreed-upon position on the globe that geographers have designated a starting point and is located at the Royal Observatory in Greenwich, England.