Camera lenses are complex beasts. Camera lenses are composed of a number of lens elements fitted together in a tube called a lens barrel. Some lenses have up to 15 seperate ground glass elements inside them which is why some lenses cost more than the camera body.
Serously 15? Why does a camera need 15 lenses?
Well we are dealing with light, which is a stranger thing than we often realize. For example, we have all seen a prism split a beam of light into a rainbow. The split happens because different wavelengths (colors) of light refract at slightly different angles in a given prism. A lens also acts as a prism, so the red, green, and blue parts of the image will not be equally in focus at the sensor without some fancy footwork on the part of the lens manufacturer. This problem (known as longitudinal or transverse chromatic aberration) is the curse of cheap lenses when it results in tiny halos of color (typically purple) at the edges of a subject. The better the lens, the less this sort of problem is apparent.
When you first buy a digital SLR camera you will probably get an 18-50 lens with it. This is an all purpose lens that can act as a basic all purpose tool.
However, these basis lenses try to do the job of both a zoom lens and a wide angle lens in one. As a result they almost always end up being a compromise. The quality of shot you get from a fixed range lens is going to be better. Alongside chromatic abberation these fixed lenses are also better at reducing: Geometric distortions, Spherical aberration, Comatic aberration, Lens contrast, Flare, Vignetting and Bokeh.
You made that last one up didn't you?
Nope, 'Bokeh' is the a Japanese word for fuzzy and is used in photography to mean the quality of out-of-focus areas in a photograph. Some lenses produce bright, hard-edged shapes in the out of focus areas which distract the eye, ruining the shot. Of course a few minutes of photoshopping can fix bad bokeh, but it is always better to not have the problem in the first place. And the way to avoid bad bokeh is to get a decent set of fixed lenses.