While there can be many subtle differences, the primary difference between interior and exterior paints is in their choice of resin. You will remember that the resin is what binds the pigment to the surface. In an exterior paint, it is important that the paint can survive temperature changes and be exposed to moisture. Exterior paint also must be tougher and resist peeling, chipping, and fading from sunlight. For these reasons, the resins used in binding exterior paints must be softer.
For interior paint where the temperature is not a problem, the binding resins are more rigid, which cuts down on scuffing and smearing.