In geography, longitude refers to the imaginary lines that bisect the globe through the North and South Poles (the ones that run vertically, as opposed to the lines of latitude that run horizontally, parallel to the equator).
If you state your exact longitude, you are describing how far east or west you are of the Prime Meridian, the imaginary line of longitude which runs through Greenwich, England. When you add your latitude, anyone with a map can pinpoint exactly where you are in the world. If you're standing at a latitude of 40°41.3'N and a longitude of 74°02.7'W, for example, you are at the Statue of Liberty.
