Histograms are used to determine whether a picture is over- or underexposed but should not replace looking at your image on the LCD after shooting. They merely provide additional information. Sometimes it is hard to tell from the preview image if certain areas are too bright or slightly too dark, the histogram should help in these instances.
The histogram plots brightness (darkest on the left, brightest on the right-hand side of the x-axis). There is NO ‘ideal’ shape to the graph. Some shots will naturally produce more brighter tones than others (eg. shooting a model in a studio with a white backdrop) & visa versa. However, to get the best tonal range the histogram should be vaguely bell-shaped.
In digital processing, overexposure is difficult to correct at the editing stage. Therefore, with high contrast subjects it is better to have an image that is stacked to the left as it’s possible to recover underexposed areas within image manipulation software (eg. Photoshop).