Difference between dBm & dB?

DBm is the unit of absolute power (0dBm = 1mW), while dB expresses a relationship between two powers: for example we can say that the power amplifier is 47dBm (ie 50W), which is an absolute value, whereas if we speak of its gain which is 20 dB for example, this corresponds to a ratio between input power and output power of 100.