Final answer:
The Roentgen (R) is a measure of radiation exposure, while the Roentgen equivalent man (rem) is a measure of the actual damage done by radiation to biological tissues.
Step-by-step explanation:
The Roentgen (R) is a measure of radiation exposure used to describe quantitative exposure. It measures the amount of energy absorbed by dry air. However, when assessing the effects of radiation on biological tissue, the roentgen equivalent man (rem) is a more relevant unit. The rem takes into account the dose in rads multiplied by the relative biological effectiveness (RBE), which measures the relative damage done by radiation.