  Discrepancies have emerged between the predictions of standard cold dark
matter (CDM) theory and observations of clustering on sub-galactic scales. Warm
dark matter (WDM) is a simple modification of CDM in which the dark matter
particles have initial velocities due either to their having decoupled as
thermal relics, or having been formed via non-equilibrium decay. We investigate
the nonlinear gravitational clustering of WDM with a high resolution N-body
code, and identify a number of distinctive observational signatures. Relative
to CDM, halo concentrations and core densities are lowered, core radii are
increased, and large halos emerge with far fewer low mass satellites. The
number of small halos is suppressed, and those present are formed by `top down'
fragmentation of caustics, as part of a `cosmic web' connecting massive halos.
Few small halos form outside this web. If we identify small halos with dwarf
galaxies, their number, spatial distribution, and formation epoch appear in
better agreement with the observations for WDM than they are for CDM.
