A cell phone's SAR, or its Specific Absorption Rate, is a measure of the amount of radio frequency (RF) energy absorbed by the body when using the handset. All cell phones emit RF energy and the SAR varies by handset model.
For a phone to receive FCC certification and be sold in the United States, its maximum SAR level must be 1.6 watts per kilogram. In Europe, the level is capped at 2 watts per kilogram, while Canada allows a maximum of 1.6 watts per kilogram.
Keep in mind that it is possible for the SAR level to vary between different transmission bands (the same phone can use multiple bands during a call), and that different testing bodies can obtain different results. Also, it's possible for results to vary between different models of the same phone--as in the case of a handset that's offered by multiple carriers. Different manufacturers also often have different methods for testing; some hold the phone 2cm away from the body, while some recommend holding it at around 25mm away.
Research abounds, but there still is not conclusive or demonstrated evidence as to whether cell phones cause adverse health effects in humans. While some studies have found a possible link between long-term (10 years or longer) cell phone use and brain tumors, decreased sperm count, and other ailments, other research has found no such effects. The science will continue, and we will continue to monitor the results, but it can take years of exhaustive research before studies actually prove anything.
If you're concerned about limiting your SAR exposure, you can take a few easy steps. You can text instead placing a voice call, use a speakerphone or headset whenever possible, and carry your phone at least 1 inch from your body (making sure the antenna is facing away from you). If you're pregnant, you should avoid carrying a phone next to your abdomen. Some researchers also caution against using your phone in areas with a weak signal since phones emit more electro-magnetic radiation during those times. Children, who have smaller and thinner skulls, should limit cell phone use, and people of any age should not sleep with an active phone next to the bedside or under the pillow.
Buying a phone with a lower SAR may make you feel more comfortable, but there's no guarantee that it is inherently safer. Also, during a call the phone may never reach the listed SAR and the SAR can change constantly depending on several factors.