1. dB is used to quantify ratio between two intensity or power values while dBm is used to express an absolute value of power.
2. dB is a dimensionless unit while dBm is an absolute unit.
3. dB is relative often relative to the power of the input signal while dBm is always relative to 1 mW signal.
Read more: Difference Between dB and dBm | Difference Between | dB vs dBm http://www.differencebetween.net/science/difference-between-db-and-dbm/#ixzz3xlmiLNfR