Some nuclei are unstable. An unstable nucleus becomes more stable by giving out radiation. The radiation can be an alpha particle, a beta minus (β–) particle, a beta plus (β+) particle, a gamma ray or a neutron. A beta plus particle is also called a positron.
Alpha particles, beta minus particles, positrons and neutrons are particles. Gamma rays are electromagnetic radiation, not particles. Each of these types of radiation is emitted from the nucleus of an unstable atom.
The emission is a random process. You cannot predict when one particular nucleus will decay, and you cannot tell which nucleus in a sample will decay next. Nuclei can only be given a chance of decaying in a certain time, and the decay of one nucleus does not affect the others.