I came across the three act plot structure about a year ago and I have to say I'm a bit of a convert. It's roots come from plays and drama, which is why there is so much crossover to screenplay writing.
In Act 1, we meet the main characters, find out how the world changes for the MC, and what thrusts them into a new experience.
In Act 2, we see development of the characters and what brought them there. The conflicts are brought to a breaking point and then...
In Act 3, we see the climax and then the denouement (or resolution).
Here are some links that have more info about this structure.
Wikipedia for a basic run through.
Michael Hauge takes it to a six-part structure.