Thank god for wiki. Sums up all those screenwriting books and blogs I was reading last year nice and quick. Straight copy paste from the screenwriting entry.
Syd Field's Paradigm
Screenwriting guru Syd Field wrote the seminal book Screenplay, and posited a new theory, which he called the Paradigm. Plot Points are important structural functions that happen in approximately the same place in most successful movies, like the verses and choruses in a popular song. Here is a current list of the major Plot Points that are congruent with Field's Paradigm:
Opening Image: The first image in the screenplay should summarize the entire film, especially its tone. Often, writers go back and redo this as the last thing before submitting the script.
Inciting Incident: Also called the catalyst, this is the point in the story when the Protagonist encounters the problem that will change their life. This is when the detective is assigned the case, where Boy meets Girl, and where the Comic Hero gets fired from his cushy job, forcing him into comic circumstances.
Plot Point 1: The last scene in Act One, Turning Point One is a surprising development that radically changes the Protagonist's life, and forces him to confront the Opponent. In Star Wars, this is when Luke's family is killed by the Empire. He has no home to go back to, so he joins the Rebels in opposing Darth Vader.
Pinch 1: A reminder scene at about 3/8 the way through the script (halfway through Act 2a) that brings up the central conflict of the drama, reminding us of the overall conflict. For example, in Star Wars, Pinch 1 is the Stormtroopers attacking the Millennium Falcon in Mos Eisley, reminding us the Empire is after the stolen plans to the Death Star R2-D2 is carrying and Luke and Ben Kenobi are trying to get to the Rebel Alliance (the main conflict).
Midpoint: An important scene in the middle of the script, often a reversal of fortune or revelation that changes the direction of the story. Field suggests that driving the story towards the Midpoint keeps the second act from sagging.
Pinch 2: Another reminder scene about 5/8 through the script (halfway through Act 2b) that is somehow linked to Pinch 1 in reminding the audience about the central conflict. In Star Wars, Pinch 2 is the Stormtroopers attacking them as they rescue the Princess in the Death Star. Both scenes remind us of the Empire's opposition, and using the Stormtrooper attack motif unifies both Pinches.
Plot Point 2: A dramatic reversal that ends Act 2 and begins Act 3, which is about confrontation and resolution. Sometimes Turning Point Two is the moment when the Hero has had enough and is finally going to face the Opponent. Sometimes, like in Toy Story, it's the low-point for the Hero, and he must bounce back to overcome the odds in Act 3.
Showdown: About midway through Act 3, the Protagonist will confront the Main Problem of the story and either overcome it, or come to a tragic end.
Resolution: The issues of the story are resolved.
Tag: An epilogue, tying up the loose ends of the story, giving the audience closure. This is also known as denouement. In general, films in recent decades have had longer denouements than films made in the 1970s or earlier.
The sequence approach
The sequence approach to screenwriting, sometimes known as "eight-sequence structure", is a system developed by Frank Daniel. It is based in part on the fact that, in the early days of cinema, technical matters forced screenwriters to divide their stories into sequences, each the length of a reel (about ten minutes).
The sequence approach mimics that early style. The story is broken up into eight 10-15 minute sequences. The sequences serve as "mini-movies", each with their own compressed three-act structure. The first two sequences combine to form the film's first act. The next four create the film's second act. The final two sequences complete the resolution and dénouement of the story. Each sequence's resolution creates the situation which sets up the next sequence.