In many of the writing tips you'll see references to Act 1, Act 2, Act 2a, Act 2b and Act 3. Not to mention Beginning, Middle and End, Midpoint, Crisis and Climax. This page is a (currently rough & brief) explanation to give some context to those references.
Three Act Structure
The structuring of a story by Acts is a legacy from the theater, which, for hundreds of years was the only way that storytellers could tell their stories to the masses. The most basic Three Act Structure splits the story into the following:
Act 1/Beginning - Setup/Introduction (location & characters)
Act 2/Middle - Confrontation/Conflict (character learns about the problem and starts fumbling his way towards solving it)
Act 3/End - Resolution of problem (& Happy Ever After - if applicable!)
Act 1 and 3 each have roughly 1/4 of the story, while Act 2 has 1/2.
Four Act Structure
However even the best storytellers can't keep someone's attention when their bladder is sending urgent signals to their brain. Plays were forced to stop in the middle, allowing their audience to refresh themselves. Thus the Three Act Structure became four:
Act 1 - Beginning (25%)
Act 2a - Middle (part 1) (25%)
***Break***
Act 2b - Middle (part 2) (25%)
Act 3 - End (25%)
Midpoint
Thing is, humans are social creatures. Having refreshed themselves, a drink in one hand and good company to gossip with, the attractions of the story diminish. They have no urgent need to return to their seats when the play starts again.
So writers introduced the Midpoint. Right before the break they started to include a Twist that turned everything the audience had thought it knew on its head. It was a huge hook, hanging over the audience so that they couldn't wait to get back into the theatre.
Crisis
aka The Dark Point
The is the point in the story (usually placed at the end of Act 2) when it seems like all hope has been lost. The Mentor or Guide may have died, the Protagonist appeared to have died. But some spark of hope: a piece of new information, the Hero "reborn", a new weapon - something spurs them on into Act 3.
Climax
This is the big showdown in the middle-end of Act 3. This is when the protagonist faces their biggest fear and usually wins (but sometimes loses) decisively.
For a thorough analysis of Acts 1-3, see:
http://thedarksalon.blogspot.com/2008/10/first-act-story-structure-cont.html
http://thedarksalon.blogspot.com/2008/10/story-structure-elements-of-act-two.html
http://thedarksalon.blogspot.com/2008/11/story-structure-act-two-part-two.html
http://thedarksalon.blogspot.com/2008/12/elements-of-act-three-part-1.html
http://thedarksalon.blogspot.com/2008/12/elements-of-act-three-cont-elevate-your.html
