Radioactivity is the process by which nucleus loses energy by emitting particles or electromagnetic radiations. This happens due to nucleus instability, and after radioactive decay we end up with nucleus with higher binding energy or more stable nucleus, in simplest terms. There can be natural radioactivity (occurring naturally on its own) and induced radioactivity (induced in laboratories).
The radiations emitted may include Alpha (`alpha` ), Beta (`beta` ) or Gamma (`gamma` ) radiations. Alpha radiation consists of a stream of alpha particles (helium with atomic mass of 4 and charge of +2). Beta radiations consist of streams of electrons, and gamma radiation consists of a stream of high energy photons (with a short wavelength).
An example of radioactive decay is nuclear fission of uranium-238, which results in generation of thorium-234 and an alpha particle.