A change is generally a slower pitch. An off-speed can be a curve, drop or even a rise thrown slightly slower than the faster version. An off-speed can be a person's change.
Every person is going to have a different grip, release, feel, speed and look to their pitch. Experimentation is what helps find the best mix to get the best result. Deception is the ultimate goal.
One of DD's coaches told her that if she can't change the speed of any pitch, she won't make it in college ball. So she practiced changing speeds of each pitch, 100% speed, 75% speed, and 50% speed. Obviously 50% speed is worthless but it was part of practice. Anyway, she incorporates this all the time now. I think it's cool. Just not sure about every college pitcher needing to have this as part of their artillery. I know this doesn't answer your question (others already have)...just felt I had to add to it.