It’s a lens hood! Many lenses come with them when you purchase them, however, some do not. I highly encourage using them for several reasons. I use them almost all the time! Not only do they eliminate lens flare and unwanted extra light (which means your photos taken outdoors will have truer colors and saturation with it on!), but they are an extra layer of protection of your lens. I always use a UV filter on my lenses, which is another layer of protection, but a lens hood is the first layer of protection. I once read a story about how a 220 lb photographer fell forward with camera in hand directly onto cement. The lens hood broke, but the lens and camera body were undamaged. He replaced the lens hood for $8. It really is smart to use them! Now, if you want lens flare in your photo, just remove the lens hood! 🙂
Some people do not like them because you have to remove the hood and invert them when storing the lens. Also, they can produce a slight vignetting that some people do not like. I do find either of these things to be an issue for me, so I use it almost always. It’s a personal preference, as all things are, but to me, it’s worth using.
If you have any other thoughts to share on using a lens hood, please respond for others to read!!!
Here’s a few articles on lens hoods:
http://www.cambridgeincolour.com/tutorials/lens-flare.htm (this article shows diagrams regarding lens flare and talks about the different types of lens hoods)