Driven and Driving Dimensions
Dimensions in AutoCAD are called driven or reference dimensions. A driven dimension is controlled by the geometry, and it reflects the actual value of the geometry that is referenced by the dimension. If you stretch a line, the dimension attached to the line will update to the new value. The only reason for a dimension on a typical AutoCAD drawing is to convey the value of a feature or part to the person who is going to build it.
The process that happens in Inventor is different from that of AutoCAD. In case of Inventor you create a 2D Sketches and then add various constraints such as Horizontal, Vertical, Parallel to further define the sketch. Adding geometric constraints allows line work to adjust in a predictable and desirable manner and helps control the overall shape of the sketch. When the geometric constraints are in place, you add parametric driving dimensions to the sketch geometry. By changing the value of the geometry, you change or drive the size of the sketch object. This is the reason that Inventor Dimensions are far more powerful than the AutoCAD Dimensions.
Hope this helps you learn more and better about dimensions . . .