A crop sensor is used to refer to a sensor that is smaller than 35mm film (24 x 36mm). The field/angle of view (FOV) depends on both the focal length of the lens and the size of your sensor. If you have a smaller sensor, the FOV will be smaller.
If you have a 40D, the FOV you get on your camera with a 16mm lens with be roughly the same as the FOV you get with a 16x1.6=26mm lens on a 35mm film camera.
I think that people tend to naturally associate a specific focal length to a specific FOV, which probably stems from the old days of film cameras. However, with digital cameras, you have sensors with different sizes (and typically smaller than 35mm film, to save costs).
To get a wider FOV on a 40D, you might want to consider the Canon EF-S 10-22mm f3.5-4.5 USM.