In beta decay an unstable nucleus decays to a lower-energy state by emitting either an electron (beta particle) or a positron (exactly like an electron, but with a positive charge...a form of antimatter...but also referred to as a beta particle). When a nucleus emits an electron in beta decay one of its neutrons is converted into a proton, causing the resulting nucleus to have almost the same mass but more positive charge. When it emits a positron, a proton is converted into a neutron. The high energy beta particle, usually between a few hundred and a million electron-Volts of energy, can pass through much more material than an alpha particle due to the fact that it interacts with fewer electrons as it does so. Beta radiation is still stopped by very thin sheets of material, however, and is easily shielded against.
Like alpha decay, beta decay has a half life and the amount of radioactive material present in a sample of unstable material which undergoes beta decay decreases exponentially over time. Unlike alpha decay, during beta decay a tiny, almost massless particle known as a neutrino is also emitted. This is the cause of beta decay's varying energy spread, the neutrino gets a fraction of the energy and momentum of the decay. Unlike the other three types of ionizing radiation, neutrinos are relatively harmless because they pass through matter almost never interacting. In fact trillions of neutrinos due to nuclear processes in the Sun pass through each of us every second, and we never even notice.
Beta decay is very important in medical imaging (Positron Emission Tomography, or PET, scans), carbon dating of materials, and material thickness measurements. Beta particles are also used radiation treatments for certain cancers like eye and bone cancer. Biology research also utilizes beta decay as a tracer for biological processes.