Personally, I have a few things to add to wharfinger's entry, though I know this is not a bbs.
Firstly, a lot of Fender twins are 100 watts, and most of the rest are 135 watts. This brings me to the division between the two. I'm guessing (with a reissue here) that most of the CBS and later models that are Blackfaces are 100 watts. Blackface twins were made throughout the entirety of the Twin Reverb run, and are what are being reproduced today. The difference, beside the output wattage, is that the Blackfaces do not have a master volume knob.
The other version, the Silverface, was produced various years during Fender history. These had a shiny metal face instead of a black plastic one, and both inputs ran through a master volume pot, which pulled out to add a gain boost. These amps also put out 135w, and were used by modern day guitar heroes like Kurt Cobain. the good thing about the minor gain boost is that it added enough growl to the signal that also heavily used effects such as Ibanez TS-9 Tube Screamers (which don't provide a large amount of overdrive) don't sound like pussy cats.