A type of digital signal processing that produces a continuous wash of echoing sound, simulating an acoustic space such as a concert hall. Reverberation contains the some frequency components as the sound being processed, but no discrete echoes.
GAIN: the amount of boost (regulates the amount of force / magnitude of a signal.
There's two definitions for you. I'm not sure what else is on your amp (other than the basic volume/eq controls) so I'll leave it at this. As for the knobs on your guitar, there are usually three or four knobs (varies between guitars)... One is a volume pot and the others are to adjust the tone (depending on where you're set on the selector switch).