The decibel is logarithmic because human hearing is logarithmic: we perceive a tenfold increase in sound power as roughly twice as loud. Common references: 0 dB SPL (threshold of hearing), 30 dB (whisper), 60 dB (conversation), 85 dB (OSHA 8-hour safe limit), 100 dB (live concert), 120 dB (threshold of pain).
In digital audio, dBFS (decibels Full Scale) measures signal level where 0 dBFS is the absolute maximum. Recording at −18 to −12 dBFS leaves healthy headroom. dB SPL (Sound Pressure Level) measures acoustic loudness. dBu and dBV measure electrical signal levels.