There are three types of radioactive emissions:

• Alpha - the least penetrating form of radiation, can be stopped with a piece of paper or a few inches of air. Alpha rays are the nucleus of a helium atom, and are produced by certain radiactive materials such as thorium and uranium.
• Beta rays are more penetrating than alpha rays, and can be stopped by a few millimeters of aluminum or other metals. They are very fast moving electrons.
• Gamma rays are the most penetrating form of radiation. Depending on their energy, they can travel through up to several inches of steel, and hundreds of feet of air. They are usually produced in conjunction with either alpha or beta rays.

A geiger counter works by detecting the ionization produced by a radioactive particle. Each time a particle of radiation is detected, the counter records this event. The number of events recorded over a period of time indicates the amount of radiation present. Often this is done over one minute intervals, resulting in the familiar "counts per minute" or CPM. The higher the CPM, the higher the radiation levels.

• You can read a more in depth description of how a geiger counter works here at wikipedia.
• More information on Gamma rays can be found here on wikipedia.

Background radiation can be emitted from a number of sources. There is man made such as nuclear weapons testing and waste from nuclear power generation. Natural background radiation such as Radon gas is found in geological formations such as granite and permeates the ground emitting alpha particles which in some area's of the Southwest, like Cornwall and Scotland are a major concern. Radon is colourless, odourless and is present because of the decay of naturally occurring uranium in the earth.

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