acts of film come from the theater, after all, both plays
and films are drama.
a film has three acts, cf. beginning, middle, end. Having
three acts is not a rule, but a structural
division. It is used since it functions well and helps makers
keep the film contents together. There can quite as easily
be two, four or five acts.
the end of each act there is a
plot point (turning point), bringing dynamics into the
film, in a way hurling the story forward towards to resolution
of the last act.
units shorter than an act are called sequences