What's the best explanation of what Story Points are?

Question

We're starting to use Story Points here for our Agile development but I find it hard to explain and also can't find any definitive answer to what they are. The best thing I can do is point to other sites (like http://blog.mountaingoatsoftware.com/tag/story-points) and give some vague generalization of what they are. I'm looking for a good explanation with some examples of use that would be helpful for others to use. Are there any good resources for explaining story points?