You may have heard about the "1.6 crop factor" in regards to the Rebel XSi (and other) cameras. This crop factor comes into play because, although the Rebel XSi has good-sized sensor at 22.2 mm x 14.8 mm, this sensor size is smaller than the sensor on a 35 mm camera (the size of which is 36 mm x 24 mm). The 35 mm camera frame size is often referred to as "full frame" sensor size. The result is that the larger sensor size on a 35 mm camera brings more of a subject into the image than the Rebel XSi at the same distance with the same lens. What this means is when you put a lens on the Rebel XSi, you are going to see less of your subject than if you put that same lens on a 35 mm camera and stand in the same place.
How much more does the Rebel XSi "zoom in" to your subjects? This is answered by the crop factor.
The crop factor is defined in terms of a camera's sensor size in relation to the size of a 35 mm camera's sensor size.
The crop factor, CF, is defined
CF = diagonal(35 mm image sensor) / diagonal(camera's image sensor)
A 35 mm image sensor is 36 mm × 24 mm The Rebel XSi has a sensor size of 22.2 mm x 14.8 mm. The length of a diagonal of a rectangle is defined as the square root of the sum of the squares of the sides. Thus, the CF for the Rebel XSi is
CF(RXSi) = sqrt(362 + 242) / sqrt(22.22 + 14.82)This is rounded to 1.6 for an approximation.
What this means is that if you put a 50 mm lens on a Rebel XSi, the area that you are going to see in the viewfinder is going to appear just as if you picked up a 35 mm camera with a lens of 1.6 * 50 = 80 mm while standing in the same spot.
Say your friend has a 35 mm (or full-frame) camera and you have a camera with a 1.6 crop factor. Say your friend stands at 10 meters from the subject to get a photo. Assuming you both have the same focal length lens, to get the same area as your friend in your camera's frame, you would stand at 16 meters away from the subject.
Note that the Rebel XSi doesn't change the focal length of the lens. A 50 mm lens on an XSi is still a 50 mm lens. All that changes is how much of the subject is captured by the image sensor. With the Rebel XSi, you are going to see less of the subject in comparison to a full-frame camera with the same lens on it. So in comparison to a full-frame camera, your image size is the same as if the full-frame camera had a 80 mm lens on it. It doesn't mean that your 50 mm is magically an 80 mm lens! It is just a convenient way to do comparisons to what has long been a popular image size (35 mm).
Please note: you don't have to multiply these numbers out and worry about them. They are used for comparing the Rebel XSi to a full-frame camera. I actually don't think about this, but just develop a feel for how much of my subject will be in my viewfinder if I stand at a given distance. I know that if I used a full-frame camera, my experience will be different for the same focal length lenses--I would likely want to stand too far away (1.6 times) from having gotten used to a camera with a 1.6 crop factor.