Thief, Thief 2 and Theif 3 are some of the best games ever made. The enviroments and story are great and think the sneaking is a blast.
I think the majority of the levels involve finding an object, or place, ect. I guess you can consider the first levels good indications of what the game is about. The levels are of different complexity and difficulty. Also sneaking past humans is different then sneaking past zombies which is different then sneaking past the "robots" in Thief 2, ect.