A Closer Look

After some research, I discovered that there are 3 types of radiation:
Alpha Radiation
Beta Radiation
Gamma and X Radiation

Here is what http://www.hps.org/publicinformation/ate/faqs/radiationtypes.html says about each one:

Alpha radiation is a heavy, very short-range particle and is actually an ejected helium nucleus. Some characteristics of alpha radiation are:

• Most alpha radiation is not able to penetrate human skin.
• Alpha-emitting materials can be harmful to humans if the materials are inhaled, swallowed, or absorbed through open wounds.
• A variety of instruments has been designed to measure alpha radiation. Special training in the use of these instruments is essential for making accurate measurements.
• A thin-window Geiger-Mueller (GM) probe can detect the presence of alpha radiation.
• Instruments cannot detect alpha radiation through even a thin layer of water, dust, paper, or other material, because alpha radiation is not penetrating.
• Alpha radiation travels only a short distance (a few inches) in air, but is not an external hazard.
• Alpha radiation is not able to penetrate clothing. 

 Beta radiation is a light, short-range particle and is actually an ejected electron. Some characteristics of beta radiation are:

• Beta radiation may travel several feet in air and is moderately penetrating.
• Beta radiation can penetrate human skin to the "germinal layer," where new skin cells are produced. If high levels of beta-emitting contaminants are allowed to remain on the skin for a prolonged period of time, they may cause skin injury.
• Beta-emitting contaminants may be harmful if deposited internally.
• Most beta emitters can be detected with a survey instrument and a thin-window GM probe (e.g., "pancake" type). Some beta emitters, however, produce very low-energy, poorly penetrating radiation that may be difficult or impossible to detect. Examples of these difficult-to-detect beta emitters are hydrogen-3 (tritium), carbon-14, and sulfur-35.
• Clothing provides some protection against beta radiation. 

 
Gamma radiation and x rays are highly penetrating electromagnetic radiation. Some characteristics of these radiations are:

• Gamma radiation or x rays are able to travel many feet in air and many inches in human tissue. They readily penetrate most materials and are sometimes called "penetrating" radiation.
• X rays are like gamma rays. X rays, too, are penetrating radiation. Sealed radioactive sources and machines that emit gamma radiation and x rays respectively constitute mainly an external hazard to humans.
• Gamma radiation and x rays are electromagnetic radiation like visible light, radiowaves, and ultraviolet light. These electromagnetic radiations differ only in the amount of energy they have. Gamma rays and x rays are the most energetic of these.
• Dense materials are needed for shielding from gamma radiation. Clothing provides little shielding from penetrating radiation, but will prevent contamination of the skin by gamma-emitting radioactive materials.
• Gamma radiation is easily detected by survey meters with a sodium iodide detector probe.
• Gamma radiation and/or characteristic x rays frequently accompany the emission of alpha and beta radiation during radioactive decay.