Lenses are some of the most important (and expensive) investments a photographer can make. The quality and build of the lens is as important as the quality and build of the camera for capturing a clean, sharp picture.
There are two key values that determine how a lens will perform:
Focal length - described in another blog post, the focal length of a lens describes how much of the scene can be captured in the photograph. e.g. 35mm
Lenses can have either a fixed focal length, or a variable focal length (zoom). Lenses with a fixed focal length are known as prime lenses. Lenses with a variable focal length are known as either zoom or telephoto lenses (telephoto lenses typically have a longer focal length).
Prime lenses tend to be cheaper than zoom or telephoto lenses and have crisper optics. However, because of convenience and the need to buy multiple lenses to cover the same capabilities of a zoom or telephoto lens, photographers typically invest in prime lenses which are specifically appropriate for the genre they shoot (e.g. an 85mm for portrait photography).
Aperture capability - the maximum aperture a lens can support. e.g. 1.2. On telephoto lenses, the maximum aperture may vary depending on the focal length of the shot. Constant aperture lenses, which retain the maximum focal length regardless of the zoom, are available but are a more expensive option.