What is a Cropped Sensor, and Crop Factor? - All of these terms relates to the size of the imaging sensor of a DSLR. In the film days, 35mm was the standard film size used for most Film SLRs. Digital SLRs with a sensor the same size as 35mm film are called a Full Frame SLRs with a Full Frame Sensor. The most popular SLRs from Canon and Nikon use a smaller Cropped Sensor. Cropped meaning the sensor is smaller than 35mm film. The graphic below shows Full Frame sensor in blue, Nikon DX sensor in yellow, and Canon APS-C sensor in green.

Crop Factor - Because cropped sensors are smaller in size than full frame sensors, lenses behave differently on these cameras. For example, Canon APS-C sensors have a crop factor of 1.6x. What this means is that a 50mm normal lens will act like a medium telephoto 80mm lens (50 x 1.6 = 80). So when choosing lenses for cropped frame cameras, you will chose a lens, that after the crop factor will suit your needs. If you want a lens that will approximate a normal 50mm lens, you would look at lenses from 28mm to 35mm.

Which DSLRs have cropped sensors - Cropped sensor SLRs are much smaller and lighter than their full frame counterparts. For canon cameras such as the Rebel T2i, T3i, 60d, and 7d all have cropped frame APS-C sensors. For Nikon their cropped frame sensors are labeled DX. Some Nikon cropped sensor models include the Nikon D3100 and D5100.

Lenses for Cropped Sensor DSLRs - Cropped sensor cameras can use most lenses available. Canon makes EF-S lenses that are specifically designed for crop frame APS-C DSLRs (not made for Full Frame). Nikon also has lenses, designated DX lenses for their cropped frame DX DSLRs. Sigma, Tamron, and Tokina also have lenses that are designed to be used only with cropped frame cameras.