Difference between Blocks and Families

In a typical CAD environment, you might create each door as a block; each of those blocks would bea separate element unrelated to any of the others. So, 20 door sizes would mean 20 floor-plan blocks, 20 section blocks, 20 elevation blocks, and 20 3D blocks if you were going to use them as liberally as we use them in REVIT. All of those in REVIT are represented with one family that can display itself in 2D and 3D and whose size, material, and visibility can be changed at any time.