In Maya, a "Driven Key" is like a "keyframe for keyframes"... You create an empty slider attribute and name it something (like "driver" lets say). You can have multiple attributes linked to that one "driver" keyframe, so that when "driver" = 1, scale can be at 100 and rotation is at 20. When "driver" = 10, you can set it so that scale = 1000, and rotation = 180. With your "driver", both scale and rotation are controlled by a single keyframe within the ranges you set.
A "Constraint" is something that has a set value, and any object constrained by it will be completely under its influence. If the constraint blend is set to 0, the object is free to move on its own. When constraint blend is set at 1, the object is completely controlled and its keyframes are overridden.
I'm trying to create a controller that can do this in AE, especially one that has multiple slider controls. It would be so cool to have something that moves around when keyframed like normal, but when a slider is ramped up, the object will interpolate to a set of values determined by the slider. Then if a second slider with a second set of values were ramped, the object would blend between these values.
Here's an animated GIF I made that simulates I'm talking about. I'm thinking "valueAtTime" and "linear" are involved, but I don't know syntax well enough to pull it off.