A beta particle is a high energy electron, which is emitted from a process involving
the weak nuclear force. This would be any nuclear interaction in which the electric
charge of a particle is changed. The most common such process is when a neutron is
changed into a proton, though there are more exotic reactions that could produce a beta
particle. If the reaction makes a particle more positive, then an electron will be
released; whereas, if the reaction causes a particle to become more negative then a
positron will be released. Whenever an electron is released it is accompanied by an
anti-neutrino (and positron by a neutrino) in order to obey lepton
conservation. Beta particles are produced in
radioactive decay (known as beta decay) and nuclear reactions
(fission and fusion) that involve the weak force, such as the proton-proton chain.
These mischievous little particles are also the ones responsible for the neat blue glow
around some radioactive materials, known as cherenkov radiation.