Cecil is CORRECT, also, other terms you might run into a

dB I = dB Isotropic (an immaginary point source that radiates
in all directions uniformly) , used for antennas.

dB W = dB, gain or loss , referenced to 1 Watt.

dB is simply the logrithmic ratio of (POWER, Voltage) to
the reference (-1 dB = a loss of 1/10 of your power, -3 dB ,
loss of 1/2 your power, and if positive, the opposite is true
1 dB = 10% power gain, 3 dB = twice the power, ect.
as info, Jim NN7K

Cecil Moore wrote:
jason wrote:

I may ask about some silly question and please pardon me
May I know what actually the unit of dbm and db is different from one
another?


dBm is referenced to a milliwatt. dB is referenced to
something else and that something else must be specified.
dBd is referenced to a dipole. A web search for "decibel"
uncovered 100 dB of information.