The researchers found other strange effects that muddied the interpretation of the studies: The rats exposed to cellphones seemed to outlive the rats in the control group, for example. There was no clear linear relationship between higher levels of cellphone exposure and more cancer at some tissue sites, and the cancer rate in the control group was lower than it should have been at other tissue sites.
The energy of electromagnetic radiation is determined by its frequency; ionizing radiation is high frequency, and therefore high energy, whereas non-ionizing radiation is low frequency, and therefore low energy. The NCI fact sheet Electromagnetic Fields and Cancer lists sources of radiofrequency radiation. More information about ionizing radiation can be found on the Radiation page.