No matter how they're actually divided, plays, like movies, have three acts. In the first act, we introduce characters that want something, who are in conflict. In the second act, they try to get it, with lots of resulting complications. In the third act, the play picks up speed and races toward the inevitable climax and resolution. Right?
Well, maybe. Here's the structure I've laid out...
1. "Anecdotal." Imagine a set of stories, all happening at the same time through your play, but all seemingly separate. But by the end of the play, they all tie together. It worked for Chekhov in The Three Sisters.
2. "Landscape." The lights come up, and we know nothing about the world we're in.
3. "Gapped." The play is told episodically, and between each episode (a scene), time has elapsed (anywhere from hours to days to years). The real movement occurs offstage, and the process of the scene, for the audience, is filling in and figuring out the changed landscape. For an interesting use of gapping (even though it's told in reverse chronology), look at Pinter's Betrayal.
4. "Process." The play gets its structure not from a story, but from some process. For example, two men are fixing a car. When they're done, the play ends.
Does this mean you should immediately stop writing plays with conventional story structures? No, but it's good to know that the three-act structure that we've all had burned into our brains isn't the only one out there, and to borrow as needed.