What is Electric charge?

Electric charge is the property of matter on which the electromagnetic interaction acts, as mass is the property gravitation acts on. It is measured in coulombs ($\mathrm{C}$) and comes in two signs, positive and negative, which cancel when added; like charges repel, unlike charges attract.

Electric charge $q$ (coulomb, $\mathrm{C}$) is a property of matter of either sign; it is conserved (the total charge of an isolated system never changes) and quantized in units of the elementary charge $e = 1.602 \times 10^{-19}\,\mathrm{C}$ (proton $+e$, electron $-e$). A macroscopic distribution is described by a charge density: linear $\lambda$ ($\mathrm{C}/\mathrm{m}$), surface $\sigma$ ($\mathrm{C}/\mathrm{m}^{2}$) or volume $\rho$ ($\mathrm{C}/\mathrm{m}^{3}$), so that a small element carries $\dd q = \lambda\,\dd l$, $\sigma\,\dd S$ or $\rho\,\dd\tau$.