I assume that you are referring to the black space above and below video recordings. The HD video standard specifies the aspect ratio of video to be 16:9, which the ratio between the width and height of the of picture. The iPad display has an aspect ratio of 4:3. Since this does not match the video standard, video doesn't fit exactly on the iPad screen. Consequently, the iPad fits the video to the screen by filling in the extra space on top and bottom with black.
Still photographs are different. There is no single required standard for photographs. Commonly used aspect ratios for digital cameras are 3:2, to match 35 mm film, and 4:3, which is based on the aspect ratio of old analog television. The iPad camera takes still photographs in the 4:3 ratio, which fit the screen exactly, and are therefore displayed without black bands.
If you record video in portrait orientation, the same issue applies, but in this case the black filler bands are on the left and right sides.