From the National Cancer Institute: Cell phones emit radiofrequency energy, a form of non-ionizing electromagnetic radiation, which can be absorbed by tissues closest to where the phone is held.
The amount of radiofrequency energy a cell phone user is exposed to depends on the technology of the phone, the distance between the phone’s antenna and the user, the extent and type of use, and the user’s distance from cell phone towers.
Studies thus far have not shown a consistent link between cell phone use and cancers of the brain, nerves, or other tissues of the head or neck. More research is needed because cell phone technology and how people use cell phones have been changing rapidly.
http://www.cancer.gov/cancertopics/factsheet/Risk/cellphones