By default, the histogram is grouped with the Navigator and Info palettes in its compact state. Not much to it, in that state. Let's see what's hiding under the hood, shall we? Drag the palette out of its group so it's floating independently. Next, click the palette menu in the top right corner. Enable expanded view, all channels view, show statistics, and show channels in color. The result will be the histogram in all its glory.
If your image has layers, you can choose the active layer or the entire image as the source for the histogram. As you make image adjustments, you can change the source to Adjustment Composite and see a before and after view of the histogram. The faded graph shows your original image and the darker graph shows the adjusted histogram.
From the channels menu you can choose to show a histogram for individual color channels, luminosity, or a combined view of all color channels in the same graph.
If you prefer the expanded view of the histogram palette, but don't want to see it all the time, you can dock it to the palette well and only pull it out when you need it for image correction.
