  LISA is a planned space-based gravitational-wave (GW) detector that would be
sensitive to waves from low-frequency sources, in the band of roughly $(0.03 -
0.1) {\rm mHz} \lesssim f \lesssim 0.1 {\rm Hz}$. This is expected to be an
extremely rich chunk of the GW spectrum -- observing these waves will provide a
unique view of dynamical processes in astrophysics. Here we give a quick survey
of some key LISA sources and what GWs can uniquely teach us about these
sources. Particularly noteworthy science which is highlighted here is the
potential for LISA to track the moderate to high redshift evolution of black
hole masses and spins through the measurement of GWs generated from massive
black hole binaries (which in turn form by the merger of galaxies and
protogalaxies). Measurement of these binary black hole waves has the potential
to determine the masses and spins of the constituent black holes with
percent-level accuracy or better, providing a unique high-precision probe of an
aspect of early structure growth. This article is based on the ``Astrophysics
Tutorial'' talk given by the author at the Sixth International LISA Symposium.
