A beta particle is a form of radiation given off by radioactive sources. It is a high speed electron emitted from a nucleus. In this form of radioactive decay a neutron changes into a proton and an electron; this electron is the beta particle.
Beta particles are stopped by a few millimetres of aluminium but can travel further than alpha particles and are therefore have a less strong ionising effect. This means they knock electrons out of air molecules forming positively charged air molecules.