- - Beginning
- - - Initial Incident
- - Rising Action
- - - Resolve to Action
- - - Failed Attempt(s)
- - - Complication/Reversal
- - - Further Attempt(s)
- - Climax
- - - Crisis
- - - Resolution
- - Ending
- - - Denouement (Closure)
While characters and worldbuilding are the flesh of a story, the plot is its skeleton. And like a skeleton, it's difficult to look at in a living story, because a good story will capture your attention and draw you in until you forget you're supposed to be analyzing it. At least, that's what always happens to me. :P The exercise of analyzing a plot is further complicated because you have to look at so MUCH material - a whole novel or RPG - to see a fully developed plot.
Another good metaphor for plot is that it is the grammar of story. A transformational generative grammar to be precise. What is that? Well... if you REALLY want to know you'll have to go read Noam Chomsky's book about it. And while you're at it, read _The Meme Machine_. But the short version is this: human beings, as a species, have pattern recognition built into our brains. Not just visual pattern recognition, but also grammatical pattern recognition, for both sentences and stories. This built-in ability is called the 'Human Language Instinct'. So basically whenever we hear, read, or play a story we are subconsiously looking for it to fulfill this pattern we have of 'the satisfying story', and it is the continuous application of this pattern by authors and audiences since prehistory that has driven the evolution of the fictional form from verbal anecdote to myth/epic myth to short story/novel.
So what are you and all your audience testing your story for? Causality and Teleology, and the way the conflict between them creates first Suspense and then Resolution.
Causality - Hopefully you know what causality is: it is the logical progression from cause to effect in a series of events. The standard example is: "A story is, 'The king died, then the queen died.' A plot is, 'The king died, then the queen died of grief.'" See? Cause and effect.
Teleology - This one, on the other hand, you may not have heard of before. While causality is observable in nature, teleology exists only in the human mind. (Unless you are a theist or animist and believe in directed evolution, miracles, manifest destiny, and that sort of thing.) Teleology is kind of like causality operating in the reverse direction - the idea that the ending needs to happen (generally to expound some sort of a moral point, the Premise of your story), and foreshadowing events are pulled into existence because the ending requires them. For example, a standard romance novel ending requires that the hero and the heroine be very in love with each other and no one else, and be ready to begin a life together. So during the course of the story struggle must occur to deepen their relationship, rivals must be shown to be inferior choices and discarded, and any social or economic barriers to the two characters being together must be surmounted.
Premise - A story has one or more premise/moral/message(s). 'Teaching' this premise to the audience is one of the author's motivations for creating the story. This is a philosophical idea that the story rhetorically supports by its plot progression. e.g. if a premise is "love conquers all", then the story has to show love conquering all, or a least a variety of obstacles. If the premise is something like, "greed leads to ruin", then you have to portray a greedy character failing to avoid ruin despite his cleverness and power, and a generous character prospering despite his gullability and/or poverty. Or of course you could combine these and show the greedy character getting closer and closer to ruin until he realizes the error of his ways, becomes generous, and thereafter prospers. (_A Christmas Carol_, anyone?) So, what moral lessons do you want your story to present to your audience?
The Conflict Between Causality And Teleology - Maybe you've heard the saying, "The night is darkest just before the dawn?" Stories operate according to this principle. The initial incident causes or reveals a problem and the protagonist acts to try to fix it, but it would be boring if he could just fix it; instead, to get a good dramatic story, the first fix attempt should backfire or should create a new and thornier problem as a side effect, like Hercules cutting off one hydra-head only to have two more spring forth. This is the conflict between causality and teleology - teleology is the desire to have a hydra with no heads (and hopefully dead), while causality is the action of cutting of the hydra's head resulting in two new heads.
Like all of human existence, it is the story of man's struggle to wrest what he needs and wants from an inertial and uncooperative universe. That right there is why all readers are naturally biased in favor of sympathizing with the protagonist rather than the antagonist or some other character - in a sense the audience (and the author) 'fights through' the plot alongside the protagonist, vicariously sharing the struggle, setbacks, and eventual victory (or defeat, but victory is a much more common ending for stories because few people have the urge to vicariously experience defeat.
Suspense and Resolution - A good plot is like sex. ;) The tension between what seems likely to happen and what the protagonist and/or the audience wants to happen inceases until the climax, where in a moment all that built up energy is released. The situation collapses into a contemplative afterglow, and the story has achieved its purpose of catharsis of tension. This is why 'dramatic tension' is considered important enough to be represented as the y-axis on the most popular diagramatic representation of plot, Freytag's Pyramid. (The x-axis is of course time, specifically the order in which the story is presented to the audience, as opposed to the story's internal chronology, which may be different.)
If you want a different perspective, second opinion, or more detailed definition of what plot is, here's a thread where I and other writing forum members attempted to define plot.
So, summing up...
Somebody Wants Something
You have no plot - nada, zip, zilch - unless and until you have one character who wants something: that's Motivation. They set out to get it and run into problems: that's Conflict. The problems get more thorny, the goal tantalizingly close to being realized or lost forever (Rising Action) until... The Climax! The problem is nullified, perhaps by being solved, perhaps by the realization that the problem can't be solved, or was misunderstood all along and the proposed 'solution' wouldn't actually solve anything. At the same time, the main character(s) have grown in some way - either by changing themselves to suit the world, or reaffirming their resolve to stay the same and change the world to suit them. Let the reader down gently by exprssing your moral and wrapping up loose ends in the Resolution and NOW you've got yourself a real story with a complete plot. Maybe it's not a great story, because greatness is a gestalt phenomenon resulting from all the artistic choices you make in creating your story; but it's definitely a story. And since you can't have a great story unless you first have a solid plot to hang it on, I'd say that's a good start.