Understand the radiation

What is radiation?
Radiation is energy that travels in the form of waves or high speed particles.

What causes atoms to be radioactive?
Atoms found in nature are either stable or unstable. An atom is stable if the forces among the particles that make up the nucleus are balanced. An atom is unstable (radioactive) if these forces are unbalanced (nucleus has an excess of internal energy). Unstable atoms are called radionuclides. The instability of a radionuclide's nucleus may result from an excess of either neutrons or protons.
An unstable nucleus will continually vibrate and contort and, sooner or later, attempt to reach stability by some combination of means:
1) ejecting neutrons and protons
2) converting one to the other with the ejection of a beta particle or positron
3) release of additional energy by photon (such as gamma ray) emission

Can unstable atoms become stable?
As the unstable nucleus emits radiation (disintegrates), the radionuclide transforms to different nuclides. The process is called radioactive decay. It will continue until the forces in the nucleus are balanced. For example, as a radionuclide decays, it will become a different isotope of the same element if the number of neutrons changes,or a different element if the number of protons changes.  
Often, when a radionuclide decays, the decay product (the new nuclide) is also radioactive. This is true for most naturally occurring radioactive materials and for some fission products. In order to become stable, these materials must go through many steps, becoming a series of different nuclides and giving off energy as particles or rays at each step. The series of transformations that a given radionuclide will undergo, as well as the kind of radiation it emits, are characteristic of the radionuclide. This is called a decay chain.

Which nuclides are radioactive?
Unstable nuclides of any element can exist. However, almost all elements that are heavier than bismuth (Bi), which has 83 protons, have an unstable nucleus; they are radioactive and are known as heavy nuclides. The commonly encountered radionuclides are iodine-129 & iodine-131, radon, cesium-137, cobalt-60, uranium and plutonium.

How long do radionuclides stay radioactive?
It depends on the type of radioactive material. The rate of decay is one of the characteristics of radionuclides, known as half-life. Half-life is the time required for the disintegration of one-half of the radioactive atoms that are present when measurement starts. It does not represent a fixed number of atoms that disintegrate, but a fraction.
For example, if there are 100 atoms of a radionuclide that has a half-life of one minute, there will be one half that number, or 50 atoms of the original radionuclide left one minute later. After the second minute, there will be 25 atoms of the original radionuclide left. The fact that this simple example points to the existence of 12.5 radioactive atoms after three minutes.
The half-life tells how quickly the radioactivity from the radionuclide will decrease.

Source: Centers for Disease Control and Emergency, U.S. Environmental Protection Agency