Histogram is graphical way of displaying certain exposure correctness. Histogram's left side shows details in shadow, middle part shows details in mid range and right side is reserved for brighter part of scene. Ideal exposure would be shown on histogram as a curve whose lowest points are in left and right sides and its highest point in the middle. Underexposed photography will be displayed so that the curve peak is shifted to the left side or that peak starts from the left.
Photography with most details in shadows
Overexposed photography when displayed, will be based on the right side of histogram. If curve has its peak in the middle but nor left nor right side of it touches the edges of histogram, then this would be non-contrast photography, and if curve is evenly distributed over the entire histogram surface then this would be over contrasted photography.
Photography with most details in bright area