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.

The two letters are then followed by a number which indicates the size of the hood mounting ring in millimetres. Some hoods fasten to the very end of the lens barrel by means of a bayonet (rotating and locking) mount and others clip further down the barrel to a small ring by means of small spring-loaded plastic clips in the hood. Generally newer lenses use the former style and older lenses the latter.
The hood size is sometimes followed by a letter from A through D. This letter indicates the hood style, and unfortunately there’s no real way of knowing what style fits what lens without looking it up, because the hood style appears to be chronological depending on when the lens was released and whether there happened to be any lens hoods already in the lineup with otherwise identical specifications but with a different shape. There is no way to tell if the hood is a standard tube or a petal shape (“perfect”) lens hood just by looking at this letter.
For example, the original EW-78 fits the 35-350 3.5-5.6L USM, the EW-78B fits the EF 28-135 3.5-5.6 IS USM, the EW-78C fits the EF 35 1.4L USM and the EW-78D fits the EF 28-200 3.5-5.6.
Finally, the hood name sometimes ends in a Roman numeral - typically either II or III. This indicates the hood version.
Generally speaking, mark II and III hoods are flocked on the interior with black anti-reflective material, like velvet. Hoods with no Roman numeral designation are typically painted flat black, but since this depends in part on when the hood was released this is not a guarantee. Some mark II hoods also have slightly more clearance around the end of the lens so that polarizing filters fit better if their predecessors didn’t.
Some hood naming examples:

ET-65 III

E indicates that the hood fits an EF mount lens.
T indicates that the hood fits a telephoto lens.
65 indicates that the hood mount is 65mm in diameter.
III indicates this is the third hood of the ET-65 series, and means in this case that the hood is flocked to reduce reflections.

This particular lens hood fits a number of Canon EF telephoto lenses - the 85 1.8 USM, the 100 2.0 USM, the 135 2.8 SF, the 70-210/3.5-4.5, the 75-300 4-5.6 and the 100-300 4.5-5.6 USM.

EW-78B

E indicates that the hood fits an EF mount lens.
W indicates that the hood is for a wide angle lens.
78 indicates that the hood mount is 78mm in diameter.
B indicates that it’s a hood of type B.

This particular hood fits the 28-135 3.5-5.6 IS USM.

ET-160

E indicates that the hood fits an EF mount lens.
T indicates that the hood fits a telephoto lens.
160 indicates that the hood mount is 160mm in diameter.

This gigantic hood fits the 600mm 4L USM IS.

Knowing this system you can figure a few things out. For example, the EW-65, ES-65 and ET-65 can all clip onto the same lenses - the only difference is the length. The EW-65 is the shallowest lens and the ET-65 the deepest, so putting an ET-65 onto a 28mm 2.8 lens is a bad idea as you’ll get vignetting unless you have a cropped image sensor on a digital camera. But you can put an EW-65 onto a 100-300 4.5-5.6 USM if you like. It won’t shield your lens as effectively as the longer hood, but it’s better than nothing. Or if you can’t find the discontinued ES-65 you can always use the EW-65 instead.