What are lens hoods used for?

I have recently brought a Nikon D3200 and I'm wondering what the purpose of a lens hood is. Do they protect the lens in weather conditions? Do they stop extra light from getting into the lens? What exactly do they do?

They prevent flare - light sources outside the lens' angle-of-view - from throwing non-image-forming light across the picture. A hood that's too deep, vignettes (darkens the corner of the image), while one that's too shallow won't work properly.

As you can imagine, they have to be made specifically for each lens - the modern, 'Petal,' type has to be a compromise, because the 18-55mm lens included with most DSLRs goes from wideangle (wide, shallow hood) to portrait (deeper, narrow one).

They help stop lens flair. That is side light from the sun or other sources showing up in your shots.
Basically they act as shade for the lens.

At a technical level, they prevent extraneous light entering into the lens. However, there's a second good reason to use one- it helps protect the lens from damage while it is fitted.