_root refers to the root timeline of the movie. _parent refers to the object that the calling object lives in. For example movie clip foo lives on the root timeline. _parent from within foo refers to root.
_level refers to the level where a movie resides. Most of the time a movie will live on _level0. If you use loadMovieNum and specify say 3 for the level to load into. Then you could reference the loaded movie as _level3.whatever.