  The full set of equations governing the structure and the evolution of
self--gravitating spherically symmetric dissipative fluids with anisotropic
stresses, is written down in terms of five scalar quantities obtained from the
orthogonal splitting of the Riemann tensor, in the context of general
relativity. It is shown that these scalars are directly related to fundamental
properties of the fluid distribution, such as: energy density, energy density
inhomogeneity, local anisotropy of pressure, dissipative flux and the active
gravitational mass. It is also shown that in the static case, all possible
solutions to Einstein equations may be expressed explicitly through these
scalars. Some solutions are exhibited to illustrate this point.
