The nucleus is held together by the strong force. The strong force is a short range (~1 fm), very strong (~100 times stronger than the electromagnetic force), attractive force that acts between protons and neutrons. Rather than focus of the force, we will focus on the potential energy well associated with this force. Conveniently, this potential well is, to a reasonable approximation, a finite three-dimensional square well.
In an attempt to maximize stability, nuclei have several options. Each of these options is a distinct type of radioactive decay. Before discussing the individual types of decays, we will first describe the general terminology used to describe these transformations.
Nuclei much more massive than iron actually have less binding energy per nucleon than iron. Thus, if some method existed by which these massive nuclei could be “broken up” into iron-sized fragments energy would be released - this is nuclear fission.
The radioactive process known as alpha decay involves the tunneling of an alpha particle (a bound state of two protons and two neutrons) through the Coulomb barrier to escape the nuclear potential.
After a radioactive nucleus decays, it often leaves behind a new radioactive nucleus, termed the daughter nucleus. This daughter then decays, often leaving behind another radioactive daughter. This chain of daughter nuclei can get very complicated, as each nucleus typically has a different decay constant, may decay by a different process, and emits a radioactive particle of different energy.