An electron is a subatomic particle with a negative charge.
Removal or addition of electrons will alter the charge of an atom and form an ion. Electrons are also involved in oxidation and reduction reactions. Oxidation involves the loss of an electron, while gaining an electron is a reduction process. Electrons are the smallest part of the atom, carrying an atomic mass roughly 1/1836 that of a proton.
Currently, there is no known substructure or components within an electron, therefore, electrons are described as 'elementary particles'.