Q) How do you tell from the histogram if the photo you've just taken is all right? Should the histogram look like a bell curve, a straight line or some combination?
A) Histograms can be confusing, but first you need to consider what the histogram represents. The histogram's horizontal scale is the measure of brightness the image sensor can resolve, from dark (on the left) to full white (on the right). The vertical scale is the number of pixels at each brightness level, with the bottom being no pixels at that brightness level and the top being many. There's no such thing as a perfect shape for the histogram because its curve depends on what the scene is like. Bell curves, straight lines or curves with multiple mountains may all be proper exposures.
The key to working with a histogram is to avoid the spikes on the far left and far right. To ensure proper exposure, avoid anything that's under- or overexposed-this means avoid having a large pixel count on the far left or far right, especially if it "clips" or is cut off at the ends. These spikes amount to detail being lost in the shadows or highlights of the image.
In some situations, the scene you're trying to photograph has too great a contrast ratio, and no exposure setting will help keep those spikes in bounds. In such a case, your options include using a fill-flash or graduated filters, or taking multiple exposures (using a tripod, of course) and merging them in an image editor.
For more information on histograms, check out "The Magic Of Histograms."