what is the difference b/w ''db '' and ''dbm''.?

Answer 1

From Wikipedia

dBm is an abbreviation for the power ratio in decibel (dB) of the measured power referenced to one milliwatt (mW). It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form. dBm (or dBmW) and dBW are independent of impedance (as opposed to dBV which is dependent, for example).

Since it is referenced to the watt, it is an absolute unit, used when measuring absolute power. It should not be confused with dB, a dimensionless unit, which is used when measuring the ratio between two values, such as signal-to-noise ratio.