PPN Formalism is shorthand for Parameterized Post-Newtonian Formalism. The term Post-Newtonian means that it is an approximation of the full gravitational theory. It is however closer to the truth than standard Newtonian gravitational theory.
The term Parameterized means that it is set up so that by changing various parameters, competing gravitational theories can be accommodated. The best-known competing theory is the Brans-Dicke theory of gravity. The downloadable pdf attachment has more information on this theory.
This formalism is mostly used for calculations in the solar system, where the gravitational field is not very strong and the velocities are not very relativistic. It does however take into account all the subtleties of general relativity (and more, due to the other theories it can support).
This page and its download are designed to give the engineer a handle on the Parameterized Post-Newtonian method, without having to know all the mathematics involved.
Engineers that work in the space industry are more likely to encounter this formalism than any other part of relativity.
The pdf document attached gives a good overview of most of the formalism, leaving out some of the more esoteric parameters that are not applicable to general relativity or the Brans-Dicke theory.