  We give a pedagogical presentation of heavy and light probe quarks in a
thermal plasma using the AdS/CFT correspondence. Three cases are considered:
external heavy and light quarks undergoing Brownian motion in the plasma, and
an external heavy quark moving through the plasma with a constant velocity. For
the first two cases we compute the mean-squared transverse displacement of the
string's boundary endpoint. At early times the behaviour is ballistic, while
the late time dynamics are diffusive. We extract the diffusion coefficient for
both heavy and light quarks in an arbitrary number of dimensions and comment on
the relevance for relativistic heavy ion phenomenology.
