Focal Length and Lenses
Focal length is not what is commonly believed, the actual length of a lens, but instead a calculation of an optical distance from the point where light rays from an object focus on the digital sensor or 35mm film surface. The focal length of a lens is calculated when the lens is focused at infinity.
The focal length of the lens describes the angle of view - how much of the scene will be captured, and the magnification - how large the object will be. The longer the focal length, the narrower the angle of view and the higher the magnification. The shorter the focal length, the wider the angle of view and the lower the magnification. This is demonstrated in the images below.
Types of Lens
There are two main types of lens available - zoom lens and prime lenses. A prime lens has a fixed focal length, such as 50mm or 135mm. Zoom lenses are able to change zoom and are expressed as a range, such as 70mm - 300mm.
There are many advantages of zoom lenses; they are much more versatile as you don't need to keep changing lens risking dust entry into the camera body and onto the sensor.
Prime lenses on the other hand usually have higher quality optics designed specifically for the set focal length. They are often cheaper as there are fewer lenses internally and fewer mechanisms, something which also makes them lighter and more compact. They also tend to have a wider maximum aperture, say f/2 or even f/1.4 which are a distinct advantage when shooting in low light conditions.