The amount of sky that you can view through a telescope is called the real (true) field of view and is measured in degrees of arc (angular field). The larger the field of view, the larger the area of the sky you can see. Angular field of view is calculated by dividing the power being used into the apparent field of view (in degrees) of the eyepiece being used. For example, if you were using an eyepiece with a 50 degree apparent field, and the power of the telescope with this eyepiece was 100x, then the field of view would be 0.5 degrees (50/100 = 0.5).
Manufacturers will normally specify the apparent field (in degrees) of their eyepiece designs. The larger the apparent field of the eyepiece (in general), the larger the real field of view and thus the more sky you can see. Likewise, lower powers used on a telescope allow much wider fields of view than do higher powers.
To demonstrate the power and field of view of a telescope, the photo on the left was taken with a 50mm lens and the photo on the right was taken with a Celestron C90 (1000mm f/11) at the same distance.
Information reproduced with the kind permission of Celestron.