Voltage level in RF power signal

What is the relation between the input power level in dBm and the voltage?

For example, if I sweep the RF power to 0 to 10 dBm (i.e mean power) how can I get the related voltage inside?

One point is to know the definition of dBm, see e.g. http://en.wikipedia.org/wiki/DBm.
The other is to be able to calculate voltage from power, which is basic electrical engineering,
V = √(P*R), R = 50 ohms usually.