dB or dBm? that is the question

In the Telecommunication world, is very use useful and common, use dB and dBm, but what is the difference between these concepts. Decibels (dB) is a rate or proportion that show us how big is the output signal comparing with the input signal, this is not a unit of mesurement, many engineers use dB indistinctly with dBm, seeing that dBm is a unit of power, because the signal is refered to 1mW, being not a rate, being a power, so for measuring power we must use dBm and never dB. And when we compare how big is the output versus input, we must use dB, for gains or losses.