I think a Shadow Master Assassin is devoted to almost solely using Shadow Discipline skills, while this includes the Shadow Master, it's not limited to just this one skill.
Oh, and in contrary to what most people belive, the main goal for a Summoner Druid isn't to sacrifice all his items on getting his skills as high as possible, because that makes a very poor build. I'm training a Summoner Druid right now, and I'll post a guide when I'm finished.