Working on rehabbing donated bikes for Christmas Anonymous, I regularly encounter bikes where the rim brake pads have hardened to the point that the brakes are ineffective. We have a stock of salvaged pads that we've generally drawn from as replacements, but that stock is getting pretty thin.
So I took a pad from a bike (a BSO with relatively conventional cantilever brakes, which is the most common config we see) and visited a local bike shop. Here they sold me a couple of pairs of Shimano BR-CT91 pads. These were a pretty close match in terms of appearance -- the actual pad is a hair shorter, and the mounting shaft lacks the ribbing of the old unit, but otherwise pretty close.
But when I went to install the pads they wouldn't go in. The mounting shaft of the new pad was ever so slightly larger in diameter than the old one, so it simply wouldn't fit.
I'm suspecting that the Shimano pads are intended for V-brakes, not cantis.
So, the question is: What different configurations of brake pads are there, and how do you tell them apart? And is there any terminology (other than obscure manufacturer's part numbers) that will distinguish one type from another? (This is particularly important if we want to order some replacement pads online.)