What is difference between dB and dBm?

When a physical quantity, such as power or intensity, is measured relative to a reference level it is expressed in decibels (dB), which is a logarithmic unit.dB =10 log (P1/P2)

dBm is used to express an absolute value of power and dBm is always relative to1 mW signal.dBm =10 * log (P/1mW)

Decibel (dB) is the ratio of two values whiles dBm is relative to milliwat and the reference for absolute power

dBm is a power unit in the logarithmic scale

dB is a ratio that represents gain or loss

dB is used to quantify ratio between two intensity or power values while dBm is used to express an absolute value of power.

Le dBm est unité de puissance absolue (0 dBm=1mW), alors que le dB exprime un rapport entre deux puissances : on peut par exemple dire que la puissance d'un amplificateur est de 47dBm (soit 50W), ce qui est une valeur absolue, alors que si l'on parle de son gain qui est de 20 dB par exemple, cela correspond à un rapport entre la puissance d'entrée et la puissance de sortie de 100.

Decibel (dB) and dB relative to a milliwatt (dBm) represent two different but related concepts. A dB is a shorthand way to express the ratio of two values. As a unit for the strength of a signal, dB expresses the ratio between two power levels. To be exact, dB = 10log (P1/P2).

In general dB =10*log10(X) where X is unitless.

Power in (dB) is a logarithmic ratio of powers

PdB=10* log10(P1/P2)

Since dB is a ratio of powers, dBm is defined by forming the ratio of the power you want to express relative to a reference power, which is 1 mW

Formally speaking,

PTdBm=10* log10(PT/1mW)

power or intensity, is measured relative to a reference level it is expressed in decibels 

dB =10 log (P1/P2)

dBm is used to express an absolute value of power  relative to1 mWdBm =10 * log (P/1mW) this is usual more simple for small quantity

more info 

http://www.differencebetween.net/science/difference-between-db-and-dbm/

As per your question "is dB relative to watts and dBm to miliwatts". dB is used in comparing two powers (supppse P1 and P2, dB =10.log(P1/P2)) and the answer is expressed in "watts" or is in "watts". dB is not relative to watts but dB is meausred in watts 

dBm is relative to1 milliwatt :

dBm =10.log(P1/*P2)

*P2 =1mWfor the equation above P1 is always measured and substituted in milliwatts and P2 is always substituted as1mWdBm is used in comparing powers relative to1 milliwatt

is dB is relative to watts and dBm to miliwatts

in terms of power it is20logp2/p1 and in terms of signal it is10logv2/v1