What are hood lens used for on a digital camera?

Stray light entering on an angle can bounce of the sides and end up hitting the film/sensor. This is most evident when shooting into or close to the sun and typically red spots will form in the image.

This condition is know as lens flare. A lens hood helps cut down on the amount of lens flare.

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Buy brand name if you are really worried. They are likely all very comparable.

To keep light sources (on the side) from making the camera focus incorrectly.

There's used to shade lenses from flare that might be caused by the sun or other stray lights.