Explanations in this section should contain no formulas, but instead colloquial things like you would hear them during a coffee break or at a cocktail party.
Concrete
The International System of Units (SI) - the modern metric system of measurement.
Gaussian - commonly used in electromagnetism.
Heaviside-Lorentz (rationalized Gaussian)
Natural units - commonly used in quantum electrodynamics and particle physics in general.
A great discussion can be found in Section 2.1 in Klauber's "Student-friendly Quantum Field Theory".