According to my paint supplier where I buy most of my paint (Glidden,
Coronado), there are differences that you should be aware of, but not
necessarily worry about. It depends on your application.
Interior paint is not very UV resistant; but it dries harder than
exterior and is more scrubable, which is the way it was designed.
Notice I said scrubable, not washable. Scrubbing allows crayon,
handprints, etc., to be scrubbed off with cleaners. I think of washing
as rinsing with my power washer with water and light cleaner. No stiff
brushing or worse.
Exterior paint strikes the balance between UV resistance and
washablility. You can wash you house from time to time for years to
remove the grime and the paint will be completely fine. However, after
the paint has been on the house for few years, try scrubbing off the
bird poop and look what it does to the finish. Not too scrubable.
I use my left over interior paint all the time for exterior primer if
it is a neutral color. I use exterior on the inside without problems
if it is a house that has no children.
In my experience, just about all interior and exterior the paints
perform the same no matter where they are used in the first year or so,
but after that you can really start to tell the difference.
If you are painting a room for adults only (no handprints or scratches
on the wall) you will be fine with the exterior.