Simplified...

High -- Still functional, just slightly intoxicated. You know, when you are just starting to feel really good but your reflexes and sense are still mostly in order.

Stoned -- Not entirely functional, very buzzed, senses and reflexes affected (how much depends on lots of things including personal tolerance, strength of the weed and just how stoned you are lol)...you know that feeling when you just want to sit on the couch, or maybe you find it hard to focus on more than one thing...When I'm stoned, I have a bad habit of lighting a black and mild cigar and just holding it, not realizing I'm doing it, or I will be just holding (not hitting) the pipe for like 10 minutes.