#2
well, overdrive distortion is the sound of tubes in amps being pushed to extreme levels, and fuzz distortion was created by a guy poking holes in his speakers
#3
Marshall amps were probably the first amps used with distortion, but they didn't have inbuilt distortion, back in the days of Plexis, you just had to crank it up until the power amp valves became overdriven (very loud sometimes, in the case of early Iron Maiden like distortion).
Examples of cranked up distortions (well, that I know of... I don't really listen to much early stuff) are Hendrix, Led Zepp and Iron Maiden (the first two albums, although I think they used some pedals).

But I have no clue how distortion circuits work, but I like it =P.
My Gear
Guitars
Cort VX-2V
Basses
Stagg BC300
Amps
Marshall JCM900 4100 DR
Marshall 1965A
Hartke HA3500 Combo