An atom is a fundamental piece of matter, everything in the Universe is made of atoms.  They are composed of a nucleus and one or more electrons bound to the nucleus. The nucleus consists of one or more protons and neutrons.
Radiation is produced by unstable atoms that have an excess of energy and/or mass (e.g. alpha particle emission when an atom emits two protons and two neutrons). An unstable atom may emit spontaneous energy and this emission is called radiation. When we talk about radiation, we generally mean ionizing radiation, in other words high energy radiation that carries enough energy to break neutral atoms of a matter into electrically charged components.  Radiation is energy travelling through space or a material medium in the form of waves or particles.

Radiation is all around us and it is naturally present in our environment. Ionizing radiation that is constantly present in our environment is called a natural background radiation. It originates from various sources, such as the sun, stars, naturally existing radioactive materials in soil and rocks (including uranium, thorium, and radium). Radiation is used in medicine as a diagnostics and treatment, power production, various industries and research.

Radioactive materials are used in science, various industry and medicine. Scientific research reactors use nuclear materials in researching properties of various materials under high radiation conditions.

In biological and biomedical research radioactive materials are used for testing new drugs, studying bone formation, etc. There are many industrial uses of radioactive materials, including material density evaluation, sterilization, inspection of impurities in materials, security, etc. Fundamental properties of nuclear materials (fissile) are used for electric power generation in nuclear reactors.

In medicine, radiation is used both as a diagnostic tool and for treatment. The most common medical procedures involve the use of x-rays for visualization of dense tissues such as bones and teeth. Radiation therapy uses high-energy radiation to shrink tumors and kill cancer cells. X-rays, gamma rays and charged particles are types of radiation used for cancer treatment. 

The exact effect of ionizing radiation depends on its type and intensity. Ionizing radiation has the ability to cause damage of live tissue cells that can be a prerequisite for cancer, genetic mutation and death. 

Radioactivity of a substance is measured in the number of nucleus that decay per unit time. The standard international unit of radioactivity is called a Becquerel (abbreviated Bq) defined as the activity of a quantity of radioactive material in which one nucleus decays per second. Very often large units, such as 1 kBq = 1000 Bq, 1 MBq = 1000 KBq and 1 GBq = 1000 MBq are used. Historically radioactivity is also measured in curie, which is a number of disintegration per second in one gram of radium - 226. 1 curie = 37 GBq.  

The Electromagnetic (EM) spectrum consists of radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma rays.  These EM radiations differ only in the amount of energy they carry. Ionizing radiation is radiation with enough energy to remove electrons from an atom, causing the atom to become ionized. People are constantly exposed to natural ionizing radiation from the Sun, starts, soil, water and food. Ionizing radiation has many beneficial uses, however long time exposure or to higher doses increases potential for health damage. Often radio waves (part of the EM spectrum) present in our everyday life (phone, computer, TV, etc.) are mistakenly considered as harmful. Radio waves are electromagnetic radiation, but because of the low-energy they fall within the category of non-ionizing and harmful effects of exposure to radio waves has not been seen.