Focal Length – “For a simple lens focused on an infinitely distant subject, the distance between the center of the lens and the point on the axis of sharpest focus. For a compound lens, the focal length is measured from the rear nodal point of the lens to the point on the axis of sharpest focus. Increasing the focal length, increases the image magnification for a given subject distance.” Pg A34 Photography by Bruce Warren

In other words, the focal length of the lens is defined as the distance from the middle of the lens to its focal point. On your lens, the focal point would be displayed on the lens barrel (eg.50mm).

There are 3 main categories for lenses, wide-angel, normal or telephoto.
They are named by describing their focal lengths. Below are popular Fixed Lenses, or lenses where the perspective or view is not adjustable. Zoom lenses offer a range of focal lengths (eg. 24mm-105mm zoom lens). The link shows a simplified zoom lens in motion (from wikimedia.org) http://upload.wikimedia.org/wikipedia/commons/e/e3/Zoom_prinzip.gif

Wide-Angle Lens = 18mm, 20mm, 28mm, 35mm
Normal Lens = 50mm, 55mm, 70mm
Telephoto Lens = 90mm, 135mm 200mm, 300mm