The primary difference between a scene and an act is length. An act is a large section of the play, usually about 45 to 60 minutes. A scene can be almost any amount of time (from three minutes to much longer), but the important point is that many scenes make up an act in a play. The act is the larger unit of time within the play which is made up of scenes.
A change in scene often signals a change in location within the play. For example, a movement from a room to an outdoor setting would indicate the end of one scene and the beginning of a new one. Acts can be made of scenes within many different locations.
A scene can indicate a period of time (whether it be minutes, hours, days or even years) having passed. As with location changes, an act can contain many changes in time (as it contains many scenes) while one scene usually covers one moment in time.
There are generally three acts in a screenplay which match the inciting moment, climax and resolution of classic dramatic structure and Freytag's Triangle. Although modern screenplays do not always adhere to this strict organization, the general movement of inciting incident, climax or peak, and then resolution or denouement, usually relate to the number of acts. Screenplays average two to 4 acts, with three being the most common number.