What is the purpose of a lens hood and when do you use one? –Hannah
I remember many a cheery late-afternoon, squatting in some awkward position in some secluded location attempting to get just the perfect angle of some perfect composition, where I had to get in an even more awkward position to hold my hand just above the end of the lens to block the sunlight from totally obliterating the beautiful quality of contrast in the image. This did happen quite often, until I got a lens hood.
Some folks use them all the time, but really, you only need them in situations where direct light is shining into the lens from the side. Therefore, they’re really handy in bright sunlight or when shooting from a stage.
There is a great explanation of the technical details at this website.