A social story is a narrative way to teach someone a social skill or routine. It often is accompanied with pictures to help individuals with Autism/Social Cognitive Differences better comprehend the narrative. It is personalized and tries to be specific in explaining what to do and why.
Here is a social story I created for a student that spent most of his day talking about the train:
Sometimes it is fun to talk about the train.
After I say 1 or 2 things, the train-talk needs to stop.
Too much train-talk can frustrate other people.
I can talk about the train later.
Other resources and information:
Carol Gray is the official Social Stories guru. She lists many examples on her site. My personal favorite is the one that assures the student that using the bathroom where the laminator is stored is okay.
The Flemington-Rarington School District (NJ) compiled many simple social stories to use with stories. These narratives are short and to the point. Stories include: bathrooming, washing hands, bus evacuation, etc. These are great texts to use with younger or lower functioning children.