What is a grad student?

At dinner, Dale and I were talking about a grad student in his lab, and Jordan asked what a grad student is.

R: A grad student is someone who has finished high school, and finished college, and decides they want to do more special research.
J: I bet you have to pay a lot of money to be a grad student.
D: Actually, the hard part is to get a school to let you in.  Once they decide that you can be a grad student, they usually help you get a job at the school, and do other things so that it is paid for.
R: To get the school to let you in, you have to show that you know a lot, and that you know how to work very hard.
A: I don't think I could be a grad student.
R: Why not?
A: Because I know a lot, but I don't know how to work very hard.