Histograms are an invaluable tool when it comes to evaluating exposure. The Canon 400D gives you the opportunity to view a histogram on the back of the camera.
A histogram is a bar graph that shows either the greyscale brightness in the image - from black- left (level 0) to white - right (level 255), or Red, Green, Blue (RGB) brightness levels along the bottom. The vertical axis displays the number of pixels at each location.
Some scenes will naturally be weighted towards one side of the scale or the other, specifically with predominantly light or dark tones. In average scenes it is best to aim for a fairly even distribution across the entire graph, and in most cases this means avoiding having pixels crowded against the far left or right side.
During image playback on the Canon 400D press the DISP button twice to display the image with a histogram. This also has a highlight alert which flashes to show areas which are over exposed. If shooting RAW capture, the LCD playback is based on the JPEG image and therefore there may possibly be a slight difference to the histogram that you view on the PC.
Useful information can be found in the Digital R.bel XTi/400D Digital Field Guide Lowrie, Charlotte K. Wiley Publishing Inc 2007
and also here