The two most widely used systems of
units in the
scientific community are the
MKS (or SI) system, and the
Gaussian system. There are several other systems of units, including for example the
natural units and
atomic units of
quantum theory (or the
Heaviside-Lorentz system of
subatomic quantum theory), but these are the most prevalent, seen in every field of physical theory.
Every system of units includes a set of base units. In the MKS system, these are meters, kilograms, and seconds. It is a surprising fact (for the uninitiated or non-scientist) that every measurable physical quantity can be reduced to some algebraic combination of these quantities.
The reason for the persistence of these different sets of units is that they give (at least superficially) different expressions for Maxwell's equations. This is not much of an annoyance as long as one is dealing with the simpler equations of electrostatics, but as soon as one becomes interested in time varying electromagnetic fields, the gaussian system becomes preferable because of the symmetric way in which both electric and magnetic contributions to the field come in.