Someone who smokes a lot of weed. Most of the time they can function in society with no problem at all. They are successful people who just like to smoke weed. However there are always those people who smoke weed all day and cannot function. Those are called losers.
Pothead is not a term to be used lightly. You cannot start calling yourself a pothead until others do first. Its like a pothead rule man.
Potheads only smoke pot. Maybe a little shrooms but no hard drugs. If a pothead does a hard drug they move into the term "druggie" which is almost a complete opposite from pothead. Potheads are cool, druggies are quite lame.
"Dude, your such a pothead."
"Tim is such a pothead, yet he gets on the High honors every Semester."