Histograms in Photoshop and on your camera can look a bit confusing but the concept behind them is actually very simple and straight forward. In this short but informative video from Envato Tuts you get a 60 second crash course on what a histogram is and the purpose that it serves. If you've ever wondered about these mysterious charts, here is what you need to know.
Histograms are an integral part of understanding what the light is doing in your image. Knowing how to read a histogram allows you to quickly sort out lighting situations, and later in post, gives you an understanding of how to create the kind of changes you'd like to see in the final product.
Its especially important in digital . E.g. google for ETL (exposure to the left) to get a deeper understanding why histogram is so important.