Before the lobby is the box office, where tickets can be bought or picked up.

The lobby is where the audience hangs out before the show begins. Snacks such as chips and popcorn can be bought but not consumed inside the auditorium. The audience returns to the lobby for the 15 minute intermission as well.
The lobby not only has concessions but a table where silk roses are sold as gifts for the performers. There are also bathrooms.

This auditorium is where the audience will sit and watch the performers. It's also where the director sits to watch the progress of the show.
This thread also includes the action on the stage, action in the seats, action in the pit (which is where the musicians play during a musical number), and action wings (which is where the performers wait for their cues to the left and right of the stage).