I recently upgraded from my kit lens to a prime (fixed) lens - which essentially means a lens at a fixed focal length (i.e. zoom) and therefore has no 'zooming' function. This particular lens is at 50mm (which on my kit lens scale of 18mm to 55mm is highly magnified). This means you have to step back to capture the same amount of image as previous compositions, at say 18mm.
The lens I purchased was the Canon 50mm f/1.8 and costs £85 on Amazon (see link).
Considered to be one of Canon's cheapest lens (evident from build quality), it's main advantage, which more than compensates plastic exterior and noisy function is the actual lens within. At an aperture of 1.8 (remembering that the kit lens only gave a maximum aperture (f number) of 3.5), you get an idea of the power of this lens. Aperture is essentially how big the whole with which you allow light through a lens is - and the lower the 'f' number, the lower the aperture. The main purpose of aperture is to allow more light to enter at a give shutter speed, in other words, with an aperture of 1.8 instead of 3.5, less light is required to take the same image. This is useful when you are in low-light conditions such as indoors (and you do not want to use a flash).
The other main and exciting feature with this lens is the ability to work with Depth of Field. I don't know enough about DoF to explain it properly (lots of the net), but in essence, it is a parallel plane (to the capture medium / camera's sensor) which is in focus with the remain sections in front and behind slows fading out of focus. Low aperture enables the plane of focus to be really narrow (i.e. shallow depth of field) giving those trademark captures with crystal clear subject and blurry backgrounds.
For example, we can see a shot I took of a goose eating bread on a winters day - the head is in focus and relatively clear compared to the background which is 'blurred' - this demonstrated the potential of this 50mm lens:
|Goose captured using Canon 50mm f/1.8 Lens|
More examples of 50mm lens images can be found on the 50mm Flickr Group.