Figure 1 illustrates the modern scientific view of a
typical material system such as a molecule or solid. The nuclei are
small, point-like objects, and we denote their locations in d=3
dimensional space as 
, 
, .... The electrons,
which have much smaller mass, move much more quickly and spread out
into ``electron clouds'' which we learn below to describe in
terms of orbitals.
Figure 1: Modern view of a molecular or solid system: nuclei (large
dots), electron clouds (grey shaded regions).
The large difference in the mass of the nuclei and electrons allows us
to consider the nuclei as nearly stationary compared to the electrons
so that we may consider total energy of the system 
for each particular arrangement of the nuclei, a
quantity which gives us a tremendous knowledge of the system. For
instance, the minimum value of E over all possible nuclear positions
gives the binding energy of the system.
The arrangement of the nuclei for which the energy is minimized gives
the equilibrium structure, including all of the bond lengths, bond
angles and, for solids, the lattice parameters. The derivatives of
E with respect to the coordinates of the nuclei give the forces
on the nuclei, which determine how
the system evolves in time. From this, we can compute the vibrational
frequencies and even the melting point of the system. These are just
a few of the things which we can compute ab initio, using only
a few fundamental constants of nature as experimental input!