Overview of R: What is it?

I have had a number of people ask me the question:  What is it?  Who uses it?  I have been surprised that some schools are not teaching it.  It seems more of a tool use for acquired needs.  And is increasingly being used to for research and sharing.  Good introductory piece about what it is, but not how to use it.  I like the cautions included, you rarely see that in a resource book.  Lots of other posts and resources for its use have been listed here.