In some materials, all of the electrons are bound fairly tightly to their atoms, except during a chemical reaction. These materials are called insulators. In others, such as metals, each atom has one or more electrons which are bound so weakly that they are able to escape their individual atoms and wander together through the material in a sort of "electron sea". These materials are called conductors. These free-flowing electrons are called charge carriers, because they can carry excess charge from one area to another, allowing electric current to flow, and sometimes carrying thermal energy as well.
There are other materials which possess charge carriers which are not electrons. For example, when table salt is dissolved in water, the atom dissolves into positive sodium ions and negative chlorine ions, both of which are allowed to move freely through the water. In cells, electric current flows in the form of positively charged ions like sodium, potassium, and calcium, in addition to negative ions like chlorine. In most cases, the sign of the charge carrier, positive or negative, has no effect on the macroscopic behavior of the system: positive charges flowing to the right are equivalent to negative charges flowing to the left.