Tweed, Brown Face, Black Face, and Silver Face not only refer to different eras and cosmentics, but also circuit topologies and tonal characteristics.
Tweed era: these amps were covered with linen luggage covering, hence "tweed." In the combos, the chassis hung down in the back of the cabinet with the controls in the back. This era lasted from the late 1940s to the very early 1960s. Tonally, these amps are more mid-rangy, and distort earlier. The circuit topologies tended not to produce as much power as later Fender amps as electronics had not advanced at this point. Iconic amps of this period are the Twin, Deluxe, Bassman, and Champ. None of these amps had reverb, IIRC. Some had tremolo.
Brown Face era: these amps had brown tolex covering with brown control panels, (although there were some that had black control panels) and either wheat or oxblood colored grills. The electronics mounted in the top of the cabinet with the controls facing front. Most of these were produced in the early 1960s. Tonally, these amps are cleaner than the earlier tweeds, but can still get great sounding OD. The circuits were evolved to the point where they were getting more power out than the tweed era amps. Leo Fender was obviously trying to get his amps louder and cleaner. I believe the later Brown Face amps were the first to have spring reverb. And there was a Brown Face stand alone reverb tank. These amps were also the beginning of piggyback amps with separate heads and cabs. The Brown Face Deluxe, Bassman, and Bandmaster are iconic amps of this era.
Blackface Era: black tolex, black control panels and silver grill cloth. They were configured a lot like the Brown Face amps with different cosmetics. This era lasted from the mid to late 1960s. These amps were louder and cleaner sounding than the previous Fenders. Fender continued to engineer his amps for more power and less distortion. I believe this was because Fender used to service amps for country players in the beginning of his career, and they had little use for distortion. Fender also collaborated with Dick Dale to build a loud surf amp, the Dual Showman. I was 100 watts, IIRC. Other iconic amps are the Twin Reverb, Deluxe Reverb, Champ, Princeton Reverb and the Bassman (at this point it was a head, and sounded nothing like the Tweed Baseman.) Tonally, these amps are more scooped sounding, less midrange.
Silver Face era: most of these were just Black Face amps with updated cosmetics, black tolex covering, silver control panels and silver grill cloth. Fender sold his company to CBS in 1965 and CBS continued to engineer amps for more power and less distortion. This era lasted from the late 1960s all the way up until 1979. CBS began to engineer "improvements" into the Fender line, increased voltages for cleaner sounds, master volumes, pull boosts, and ultra linear output. Because of this, Silver Face amps are not as well regarded as the earlier Black Face amps. Some of the amps only got cosmetic changes, like the early Silver Face Deluxe and Twin Reverb. These amps can usually be tweaked back to Black Face specs. Later amps with the "improvements" generally can't be modded to BF specs because of the extra circuitry and very high voltages.
Overall the progression was from lower powered, midrangy amps that distorted quickly, to more scooped sounding amps with higher power and less distortion.
Edit: forgot Blonde amps, but for all intents and purposes, they are similar to the Brown Face amps.
