Gamma rays
Gamma rays are photons that are emitted from the nucleus. Often an atom in an excited state will de-excite by emitting a gamma ray. Gamma rays are similar to light waves and x-rays, except they are usually much higher frequency and consequently, more energetic. This radiation has no charge, and can penetrate most matter easily, requiring lead bricks for shielding.
Common sources of radiation
"Man-made" radiation
Smoke detectors
Smoke detectors make use of the isotope Americium-241. This isotope emits alpha-particles at energies up to 5.4 MeV. The energetic alpha particles are used to ionize air. Once the air is ionized, a small current runs through it. When smoke enters the chamber, the current experiences an increase in resistance and a circuit sounds the alarm.
Coal-burning power plants
Coal is an impure fuel, and it usually contains 1.3 ppm of uranium and 3.4 ppm of thorium (not to mention arsenic, mercury, and sulfur). When coal burns, these isotopes are emitted into the atmosphere, where they enter our ecosystem. This leads to the astounding fact that the population effective dose equivalent from coal plants is 100 times that from nuclear plants.