What's a crop factor?
Most film cameras are 35mm, and that refers to the size of the recorded image on the film. Professional-level ($$$) digital cameras have "full frame" sensors that are 35mm, and therefore replicate film exactly. So-called "Pro-sumer" DSLRs usually have smaller sensors in them because they are cheaper to manufacture. The smaller sensor obviously doesn't impact the image resolution much (DSLR's with cropped sensors are now being made at 10MP or higher resolution), but they do impact the way your lenses work.
Why might I want a full frame camera?
Full frame cameras have better high ISO performance. This means, in low light when you have to increase the ISO to get a clear shot, you'll get less noise (that grainy, static-looking stuff in your photo) with a full frame than you will with a cropped sensor camera. You may get a higher quality image as well, but you won't be able to tell the difference at normal print (11x14) or on-screen sizes. As person who tries to buy the best all the time I held out for a full frame camera for a long time before deciding to go cropped instead. I've found I have no need for a full frame and feel this is true for anyone who's not making enough money from their photography to cushion the extra expense. I've sold photos taken with a point and shoot, so buying a crop sensor camera won't impact most sales if that's your aim.
How do we account for the crop factor when looking into buying a new lens?
Easy! Look up the crop factor of your camera. For most Canons (20D, 30D, 40D, 300D, 350D, 400D, Rebel, Rebel XT, and the Rebel XTi) it's 1.6. For other camera types, just google your camera's make and model along with the words "crop factor" and you'll probably get an answer right away.
Then all you have to do is multiply your crop factor times the focal length of the lens to figure out what the equivalent would be for film. So, if you're looking at a 50mm lens and you have a Canon 30D, it would act like a 80mm lens (50 x 1.6 = 80).
But if, instead, you're looking for a lens that ACTS like it's 50mm on your cropped sensor, then all you have to do is divide 50mm by your crop factor. 50 / 1.6 = 31.25. So something in the 30mm range would work out best.
Why does this matter?Most of the time it doesn't. You just use your lenses to get the photos you want. If your 100mm lens isn't long enough for you to photograph birds, you go look for a lens with a higher focal length, and you'll know that a 200mm lens will get you twice as close. None of that changes. And don't fear that your photos will be "cropped" from what you see in the viewfinder -they will not. What you see is still what you get. The only time this will matter is if you're getting advice from someone with a film or full sized sensor camera -or, if you're thinking of buying a full frame camera vs a crop. That's all no biggie :-)