What are the two main divisions of a play?

An Act is a portion of a play separated from the rest by an intermission. Originally, when plays were performed by candlelight, the breaks between acts were taken to replace or trim the candles. Acts are divided by scenes. In the French tradition, a new scene starts whenever a character leaves or enters the stage. In the English tradition, a new scene starts when the setting changes. Classically, this would only be a change in the physical setting, but in modern plays it can be a change in the temporal setting as well, where you get scenes starting "The same, one week later".