Plot Structure

Almost all stories have a beginning, a middle and an end. Similarly, plays have a three-act structure - the set-up, the confrontation and the resolution. Historically, this structure has been drawn like this:

These days, the three elements tend to be sub-divided into what is commonly known as a plot structure. The six parts can be seen as:
Exposition
Conflict
Rising action
Climax
Falling action
Resolution

This is how they are represented in a plot diagram:

Exposition
This element is where you present the main character and other characters and show how they're living before anything affects them. The reader can get to know your characters here.

Conflict
This is where something needs to happen - an unexpected crisis, a difficulty or a dilemma facing a character or characters. At this point, the tension rises, represented by the rising line in the diagram.

Rising Action
The rising action can consist of several rises and falls in tension, as the character(s) attempt to resolve the conflict.

Climax
This is meant to be the most exciting moment in the story and where the conflict is usually resolved.

Falling Action
This is where where the outcome of the climax is played out, where it's effects on the characters are shown. As the line falls, the tension decreases.

Resolution
This is the very end of the plot. It may be happy or sad. It may leave questions unanswered.

Notes:
It's a good idea to lay out your story following these principles to begin with, but there's nothing to stop you then playing around with the sequence.
For example, if you want to grab the reader straight away, you may want to begin your story nearer to a conflict and fill in the 'Exposition' as you go along.
Many films these days rearrange the chronology of events for dramatic effect. Remember, there are no hard and fast rules!