deciBell

dB = 10 log P for power
dB = 20 log V for voltage
Power ratio = 10^(dB/10)
Voltage ratio = 10^(dB/20)

        power  voltage
0.1 dB    1.02    1.01
0.2 dB    1.05    1.02
0.5 dB    1.12    1.05
1 dB      1.26    1.12
2 dB      1.54    1.26
3 dB      2.0     1.41
4 dB      2.51    1.58
6 dB      4.0     2.0
8 dB      6.31    2.51
10 dB    10       3.16
20 dB   100      10
30 dB  1000      31.6
40 dB   10k     100
50 dB  100k     316
60 dB    1M    1000
70 dB   10M    3160
80 dB  100M     10k
etc.

dB references

Voltage
dBu or dBv, reference is 0.775 VRMS
dBV, reference is 1 volt RMS
dBmV, reference is 1 mV across 75 ohms
dBµV or dBuV, reference is 1µV

Acoustic
dB(SPL) (Sound Pressure Level), reference to 20 µPa
      dB = 10 log (Sound Pressure Level / 20 µPa)
dB(PA), reference to 1 Pa
      dB = 10 log (Sound Pressure Level / 1 Pa)
dB SIL (Sound Intensity Level), reference to 1^-12 W/m²
      dB = 10 log (Sound Intensity Level / ref)
dB SWL (Sound Power Level) referenced to 1^-12 W
      dB = 10 log (Sound Power Level / ref)
dB(A), dB(B), dB(C). these are dB SIL but with different
   frequency weighing filters.
dB HL (Hearing Level) is used in audiograms as a measure of
   hearing loss

Electrical Power
dBm is db referenced to 1 mW
dBW is db referenced to 1 watt
they differ by a factor of 1000 or 30 dB

dBFS (Full Scale) reference is the max signal the amplifier
   can handle
dBTP (True peak) reference is the max peak signal the amplifier
   can handle