Canon 28 90mm
Why digital camera lens lenses different from most? ex. 18-55 mm is equivalent only 20 to 90 mm?

Before, the number of lenses 18 mm, as is already considered as a fisf eye lens. But now in the digital camera as the default lens for Canon Rebel XT, 18 to 55 mm is like a 28-90 mm Series AF target.

It's about the size of the sensor. In a film camera 35mm, of course it is a certain size (24mmx36mm I think). But digital cameras do not have to be made of the same geometry, for most compact cameras that use a smaller image sensor and place closer to the lens. Then they have to shorten the focal length accordingly. This effect is not only seen in digital cameras. If you look at medium and large format film cameras you will notice that they have very, very long focal length compared to 35 mm.