Canon lens hoods are identified by a confusing and cryptical alphanumeric code. There is a system to the hood naming, though it’s only somewhat informative. Still, if you understand how the hood naming works you can usually figure out which hoods can be interchanged with other lenses.

The first letter in a hood name is E, indicating that the hood fits a Canon EF mount lens.
The second letter is either W, S or T. W stands for Wide, S for Standard (probably) and T for Telephoto.
The letter refers to the type of lens to which the hood fits. W is for any lens wider than 50mm, S is for a 50mm lens (with a couple of odd exceptions) and T is for any lens longer than 50mm.

Aside from making the lens look longer and bigger and thus more impressive to non-photographers, lens hoods (sometimes called shades) serve two basic functions. First, they help reduce the amount of stray light hitting the surface of the lens. This is a good thing, since non-image-forming light coming into the lens at an angle results in lens flare. Lens flare can result in lower-contrast images or, in extreme cases like light from the sun, can result in big glowing blobs in the final photo. Second, lens hoods serve as physical protection for the lens - the plastic or metal tube can absorb blows that might otherwise hit the glass itself.