The Beginning
1. Character
2. Conflict/problem (the "collision idea")
3. Setting (where most newbie writers are very weak)
The Middle
4. Character tries to solve the problem
5. Character must fail (not for stupid reasons, though) and things must get worse (even better if the well-meaning actions of the character make it worse)--this is the most common plot development that beginners miss.
The End
6. Climax - character tries to solve the problem again (and either fails or succeeds--either outcome is valid)
7. Validation (shows that the story is over)
--Writers can violate these rules only if they're very good writers and know what the rules are that they're breaking. Mainstream short stories will often have no beginning and no end (or, they're implied), but these 7 points are almost mandatory with genre stories.
(Source: pilfered from http://www.io.com/~wendyw/)