Most days I am asked questions about lens hoods also known as sun shades or those tulip-looking things on the front of a lens. These devices serve very practical functions aside from making your lens look really cool.
The primary function of the lens hood is to prevent stray light just beyond the perimeter of the image from causing flares on your photos. The flares can appear as light streaks or even as a series of hexagonal or octagonal shapes caused by reflections off the aperture blades in your lens. There will frequently be many of these flares, as there can be one for each element within the lens. While the following photos won't win any prizes but they illustrate how flares can be controlled with a lens hood.
The photos were taken with a Canon EOS Rebel T1i with the commonly included Canon EF-S 18-55mm f/3.5-5.6IS lens. Both pictures were taken with the same exposure setting, same focal length and locked down on a tripod. The top image has a flare circled in black in the upper right hand corner, and the lens hood was off the lens. The bottom image was taken with the lens hood on, and the flare is gone. The big golden circle on the wall is a reflection from the big gold reflector on the right. Since that stray light came from within the frame of the photo there was no good way to eliminate it at the point of capturing the image. However a little work in Photoshop could remove it.
The lens hood will also protect the lens in a more physical way as well. Since it generally extends well beyond the front of the lens it can protect the front element from precipitation, dust and small, grabby schmutzy fingers belonging to the child whose sweet image you just captured as he tries to see the picture on the camera.
and sizes depending of the lens it fits. Telephoto lenses have longer hoods. With the narrower field of view they capture they can have longer hoods for greater flare protection.
Wide angle hoods are shorter and frequently notched out in the corners giving them something of a tulip shaped look. They need to be notched out so that they do not darken the corners of your photo. If the Canon ET-65B for the EF 70-300mm IS on the left were used on an EF-S 18-135mm IS lens that needs the EW-73B hood on the right, the image would in fact be circular. To prevent such an effect, most lens hoods attach to the lens with specific bayonet fittings. 
Some hoods have other distinctive characteristics. The rectangular Pentax hood on the left is fairly deep and it is quite wide to offer maximum protection when used on normal lenses such as the FA 50mm f/1.4 or the new DA 35mm f/2.4 lenses. The hood on the right is for the 18-55mm lenses, and it fits them all (DA, DAII, DA L and the DA WR). Most of the Pentax hoods are cleverly designed with a door so if you are using a filter with a rotating ring like a polarizer or graduated neutral density filter you will be able to rotate the ring without having to remove the hood or try squeezing your fingers inside the hood.
Finally there are variations on the above themes with regard to fittings. Some have locking rings that screw on to the hood and it is tightened when the hood has been rotated to the proper position. Some have clamp fittings that tighten with a set screw to be used with lenses that perhaps have damaged filter threads or bayonet flanges. They are also useful if a lens also has an adapter ring on the end of the lens. There are even hoods that appear to close in on the image area such as the small Samsung hood in the middle. The design does prevent flares while keeping a priority on a compact size. Lenses with such hoods are typically referred to as pancake lenses due to their extremely small and flat design.
Many lenses include a dedicated hood, but a good number of them do not. If your lenses did not include a hood, they are very commonly available at your favorite local photo specialty shop.
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