Electrons were subatomic particles that orbited the nuclei of atoms. Most atoms contained an equal number of electrons and protons; if not, then the atom had a net charge and was known as an ion. Electrons had a negative electric charge and a mass many orders of magnitude smaller than protons and neutrons – the two other principal particles of which atoms were comprised. The antimatter equivalent of an electron was a positron.
Chlorophyll gave off high-energy electrons that are picked up by other molecules, as Julian Bashir told Goran'Agar in 2372, which gave Chief Miles O'Brien time to create a plasma charge. (DS9: "Hippocratic Oath")
A telepathic message communicated to Counselor Troi contained the phrase, "One moon circles." Upon further investigation, with the assistance of Data, the USS Enterprise-D crew determined the phrase analogously referred to the hydrogen atom, in which "one electron circles one proton." (TNG: "Night Terrors")