A lens hood serves two main purposes:
1) To block stray light from getting inside the lens, usually from overhead sun. Stray light is bad because it can cause glare, lens flare, or sun spots in your image, and it can decrease contrast which can sometimes look hazy. A lens hood works a lot like a sun visor in your car.
But in most situations where there is no strong sunlight or other overbearing light source – the visible difference of a lens hood will likely be unnoticeable.
2) As a simple form of protection. A properly attached lens hood can prevent things from smashing into the front of your lens or can absorb impact if you drop it. Not the most technical application in the world, but better a smashed lens hood than a smashed front lens element.
The pros of using a lens hood certainly outweigh the cons (none really) and for that reason I personally always use a lens hood when I shoot. I consider it a good habit and it gives me a few seconds to gather my thoughts on what exactly I want to photograph before I start firing willy nilly.
A few other things to keep in mind – not all lenses include a lens hood with the lens. They are sometimes sold separately. Secondly, most lens hoods can attach forwards and backwards, so if you do want to use it for any of the above reasons make sure you screw it on with its “covering” over the front of the lens.
Now you’re one step closer to knowing (or at least looking like you know) what you’re doing with that new camera!