I think I understand, but I wish to clarify it. I have been reading about interpolation, and I come across this line in the book: " The process of filling in the blanks between keyframes is called interpolation, and the frames between keyframes are called tweens."
What I think this means is: Interpolation is the process, and a tweens are what they are called in the timeline. I would like to know if I'm right on this assumption? Thank you.

You are correct. Tweens come from "Between". Interpolation, is the mathmatical process of calculating the changes between keyframe a and keyframe b. In Poser the keys are a brighter color than the keys. You have to think of them as temporary frame because if you change the keys, the "tweens" are recalcualted. You'll have to start looking at spline, fixed and linear modes as well.

Thank you, I appreciate your help on this matter. I have been looking at the different types of interpolation. Although the only one I ever worked with is spline, the default mode. I must say this stuff can be complicated. Good thing there is a good support group here at Renderosity. Thank you again, I very much appreciate it.

Anytime. If you can, get hold of mimic, or the pose set created from mimic... you'll see how they use linear splie and break spline for facial animation.... you'll learn a lot

I will, Thank You!