What are Statistically Improbable Phrases?
They're phrases that occur more often in a piece of text than in general English. They can give you a sense of what the text is about.
The best way to understand this is to try it out. For example, to see which words occur specifically in Calvin & Hobbes, type http://s-anand.net/calvin_86.html in the box above.
You'll see all words in the URL listed alphabetically.
The big words occur more often.
The dark words are more improbable.
P.S.: Technically these are statistically improbable words. I'll modify this to incorporate phrases soon.