This is a brief introduction to general relativity, designed for both students
and teachers of the subject. While there are many excellent expositions of
general relativity, few adequately explain the geometrical meaning of the
basic equation of the theory: Einstein's equation. Here we give a simple
formulation of this equation in terms of the motion of freely falling test
particles. We also sketch some of the consequences of this formulation and
explain how it is equivalent to the usual one in terms of tensors. Finally, we
include an annotated bibliography of books, articles and websites suitable
for the student of relativity