In photography the lens is far more important than the camera body itself. You will never regret investing in your lenses because not only will it improve the quality of your images but a good lens will hold it's value.
When looking to purchase a high quality lens here are several factors to consider; focal length, aperture and price. If you were to compare the purchase of a watch to a camera lens, you could spend either $200 on a watch or $2,000 on a watch. The $2,000 watch will probably "look" cooler than the $200 watch but the truth is they will both do exactly the same thing...tell time.
However, when it comes to a camera lens spending $2,000 versus spending $200.00 will give you a completely different looking photograph. The quality will be far superior regarding sharpness and color rendition that even an untrained eye can see.
Focal Length: The focal length of a lens is defined in millimeters. This actually refers to the distance between the lens and the image sensor and doesn't translate into anything about distance from you to your subject.
Prime Lens-one focal length. These are the highest optical quality because it is only one piece of glass.
Zoom Lens-many focal lengths built into one lens. Many pieces of glass so it is important to choose a good quality zoom. The focal length is expressed by stating the minimum and maximum distance of the lens. ex. 24-70mm.
Wide Lens and Ultra Wide lenses-wider filed of view starting around 8mm-10mm up to about 24mm.
Standard Zoom Lens-medium focal length not too wide or telephoto 24mm-70mm.
Telephoto Zoom Lens-greater focal length starting around 70mm up to 600mm.
Aperture or f/stop:
This is stated on the lens in addition to the focal length and is the 'maximum' aperture value. So if the lens states f/2.8 then the lens can open as much as f/2.8-it can go smaller (up to f/22.0) but it cannot go any larger.
Variable Aperture on Zoom Lenses:
If a lens states that it is an 18-55mm, 3.5-5.6:
This means that at the 18mm focal length the maximum aperture can be 3.5; it can go smaller but it cannot go any larger(up to f/22.0)
However, at the 55mm focal length the maximum aperture will be f/5.6-it can go smaller (up to f/22.0) but it cannot go any larger.
Image Stabilizers/Vibration Reduction:
This is a feature that is built into many lenses to reduce visible shake. It is especially helpful on longer focal length lenses where shake is magnified. Always remember to shut off the IS/VR when putting the camera on a tripod, if you don't your images will actually be out of focus.
Picking the right lens for the right situation is a topic that is explored in depth in my upcoming Advanced Shooting and Critique class.