MegaHAL is a "conversation simulator": a computer program which responds in natural language to what you type, written by Jason Hutchens, who is now working with Artificial Intelligence on an exciting project to create "true" AI. It is able to "learn" what to say by observing the things which you write to it, and is therefore not limited to the English language.
A chat bot is a humanlike conversational character.
Its conversational skills and other humanlike behaviour is simulated through artificial intelligence.
It often acts as a virtual assistant and it can have its own visualisation through an avatar
or it is faceless. We expect that through the years every conversational chat bot will grow into a real virtual human.