When choosing a lens for a Digital SLR you may come across a term called the crop factor. The crop factor is a value that compares the size of a sensor on a Digital SLR to a sensor on a traditional 35mm film camera. The crop factor is important because it has an impact on the focal length of a lens attached to your Digital SLR.
Some DSLRs have a sensor that is the same size as those you find on a 35mm film camera - these tend to be among the high end and more expensive Digital SLRs. Such cameras are known as "full frame" cameras and do not have a crop factor.
The crop factor can differ between camera bodies to complicate the issue. Cheaper and entry level Digital SLRs tend to have the smallest sensors and therefore the largest crop factors. If you are buying a starter Digital SLR the crop factor is likely to be in either 1.5x or 1.6x. Always check the specification of the camera body you are considering buying to confirm the crop factor the camera has.
The focal length of any lens is described using numbers. For example, a lens may have a focal length of 28 - 70mm. The focal length described is when the lens is used with either a full frame Digital SLR or with a 35mm camera and should you use the same lens with a Digital SLR with a crop factor then the field of view of the lens changes. To illustrate, a lens with a focal length of 28 - 70mm and a camera with a crop factor of 1.5x would mean the focal length of the lens becomes 42 - 105mm. You must understand the impact this will have when choosing your digital SLR and lenses for it.
For camera users who own a number of lenses you previously used with a 35mm camera the crop factor is likely to have a significant impact if you switch to a Digital SLR that is not full frame. My advise of course is to choose a digital SLR with a low or no crop factor. The likelihood is that you will need to consider adding new wide angle lenses to your collection with the focal length starting around the 18mm mark.