More specifically, a tween is the position of an object (or line, whatever) at some frame between time "A" and time "B" -- if it takes 30 frames to move across the screen, then at the first frame it will be 1/30 across, and the next frame 2/30, at the next frame 3/30... each of these locations is a "tween," as in "in-between" (an old term from hand-drawn animation).

assuming a constant frame rate, how the tweens are spaced in 2D (or 3D0 creates the impression of varying speed.

I like the Ani library http://processing.org/reference/libraries/#animation -- very easy to use and the docs are pretty clear. The others all serve good purpose too.

kb, http://www.riftgame.com/