35mm film was a series of little rectangles, each measuring 24x36mm. I'm sure you remember them. Now we shoot digital, so there's a sensor where the film used to be. A few cameras have full-size sensors (as large as the piece of film we all remember) but the vast majority of digital cameras have a sensor that's a little smaller. The crop factor means that some of the image (compared to what it would have been on film) is not getting captured. This is somewhat irrelevant in day-to-day shooting - what you see through your viefinder is what you're going to capture. Where it matters is in lenses. Because the 35mm film size and the related jargon had become a quasi-standard, the vast majority think of a 24mm lens as wide, a 50mm lens the middle of the road, and 100mm lens as being "long" - a telephoto lens. Especially if you like to shoot wide, the crop becomes something to be aware of. See Matt's description above.