dB: Relative comparison unit.
dBm: Transmitted power. -30dBm is 1 µW. 0dBm is 1 milliwatt. 30dBm puts out 1 Watt.
dBi: Gain of an antenna as referenced to an ISOTROPIC source (point source, which does not exist).
dBd: Gain of an antenna as referenced to an DIPOLE source.
A simple dipole antenna has a gain of 2.41dBi, and a gain of 0dBd (dBi is better for advertising).