  The balance of evidence indicates that individual galaxies and groups or
clusters of galaxies are embedded in enormous distributions of cold, weakly
interacting dark matter. These dark matter 'halos' provide the scaffolding for
all luminous structure in the universe, and their properties comprise an
essential part of the current cosmological model. I review the internal
properties of dark matter halos, focussing on the simple, universal trends
predicted by numerical simulations of structure formation. Simulations indicate
that halos should all have roughly the same spherically-averaged density
profile and kinematic structure, and predict simple distributions of shape,
formation history and substructure in density and kinematics, over an enormous
range of halo mass and for all common variants of the concordance cosmology. I
describe observational progress towards testing these predictions by measuring
masses, shapes, profiles and substructure in real halos, using baryonic tracers
or gravitational lensing. An important property of simulated halos (possibly
the most important property) is their dynamical 'age', or degree of internal
relaxation. The age of a halo may have almost as much effect as its mass in
determining the state of its baryonic contents, so halo ages are also worth
trying to measure observationally. I review recent gravitational lensing
studies of galaxy clusters which should measure substructure and relaxation in
a large sample of individual cluster halos, producing quantitative measures of
age that are well-matched to theoretical predictions. The age distributions
inferred from these studies will lead to second-generation tests of the
cosmological model, as well as an improved understanding of cluster assembly
and the evolution of galaxies within clusters.
