Physics has discovered that everything is made up of atoms, and atoms are made up of protons, neutrons and electrons circling round a neucleus. Protons have a positive charge, neutrons have a neutral charge, and electrons have a negative charge. Ordinary matter is electrically neutral with the same number of protons as electrons so that the positive and negative charge equals out.
When these charges are out of balance, an atom becomes either positively or negatively charged. The switch between one type of charge and the other allows electrons to flow from one atom to another. It is this flow of electrons, or a negative charge, is what we call electricity.
So how does the charge get seprated in the first place?