Radioactive decay is the emission of nuclear radiation from an unstable radioactive nucleus.
Nuclear radiation is also called ionising radiation because the radiation once emitted in the air can hit an electron out of an atom and produce an ion (a charged atom).
Such ions are highly reactive. This chemical reactivity is the reason why radioactive radiation is so dangerous for humans and any other biological organisms.
Radioactive decay is spontaneous: it requires no external stimulation or special conditions to decay.
Radioactive decay is random. It is not possible to predict when a specific radioactive atom will decay. The radioactivity of any macroscopic object is approximately constant given that it consists of a very large number of individual atoms.
Similarly to nuclear reactions, the number of protons and neutrons (and thus of nucleons) and the total mass-energy are conserved in nuclear decays. They are both mediated by the strong nuclear force.
All chemical elements have radioactive isotopes (i.e. which emit nuclear radiation). Many of the elements with large atomic numbers only have radioactive isotopes and no stable isotopes.
A small amount of all surrounding matter (e.g. air, water, soil) is made up of naturally occurring radioactive isotopes that emit what is known as background radiation.
Because of this, a radiation detector will still register a small amount of radiation, even if not placed near a radioactive source. This must be taken into account when performing radiation measurements.