What is the "Tea Party Express"?

I don’t really know much about the “tea party,” so I don’t understand this article. Were there those who actually stood behind this guy for writing such a disgustingly racist fictional letter?

Maybe I should start with “Who is the Tea Party?” My dim understanding is it’s comprised of those insanely rabid republicans, but that’s all I know. What is their goal? How does that horrible letter fit in with what they’re doing?