Only the most expensive DSLRs use full-frame sensors - one that's about the same size as a 35mm frame of film. More typical DSLRs, like the Nikon D7000 or Canon T2i, use smaller sensors. This video shows what that means.
Mike Collins's setup compares a 5D Mark II, which has a full-frame sensor, and a 7D, which has a smaller APS-C sized sensor. Using a variety of lenses, the video illustrates exactly how the crop factor of smaller sensors affects shots - much easier than trying to explain it with wordy words. [Vimeo via Planet5D via Petapixel via ZEROFLO]