Class A or Class AB? Texture knob

[-staind-]

Ok ok, so I recently got a new amp(Peavey valveking 112) and I notice this little knob at the back that says "Texture." On one side it says Class A and the Other is Class AB.

What does all that Class [Letter] stuff mean because when I compare A, to AB I hear no difference.

[violation]

http://en.wikipedia.org/wiki/Amplifier#Electronic_amplifiers