A beta particle is an electron emitted from the nucleus of an atom that has undergone beta decay. The electron is called a beta particle as a way of distinguishing it from the electrons present in the atom's shell.
Atoms with too many neutrons can be unstable and are prone to beta decay. Beta decay occurs when one of the neutrons falls apart and releases an electron and a neutrino. Beta particles have enough energy to pass through several meters of air, and their passage through the water used to contain spent nuclear fuel rods creates an eerie blue glow called Cerenkov radiation.