[PROJECT] History Lessons

I definitely know, or at least have a general idea, what samurai are. The word "samurai" comes from the Japanese "samurai" or "samurau" - "to serve." They are something like knights, but their honor is demonstrated through actions, not just words. However, ninjas have always raised some questions for me. I know that they are special warriors who are practically impossible to defeat (you have to find them first), and they are trained in the art of combat and survival from a very young age, but who are they in essence, and how do they differ from shinobi?

And, by the way, who are ronin?