Electrons are very small "particles" (in fact they are infinitely small — they are thought to have no dimensions at all!) They have charge, mass, and spin. They form a quantum cloud around the nuclei of atoms. Since electrons are on the outside, the interaction among materials is really the interaction among electrons. All electrons carry negative charge, and since like charges repel each other, the little clouds of electrons are what are keeping your butt from sinking into your chair right now. Chemistry is mostly about electrons. Electricity is mostly about electrons. Beta particles resulting from beta decay of radioactive materials are also electrons. (For example, the decay of radioactive Carbon-14 into stable Nitrogen-14 is beta decay, and the basis for "carbon dating".)
A museum site with some history
Some more definitions about electron flow
This page supports the article "How Batteries Work"
at Science in Action