The ADSR envelope is the standard model: Attack (time from silence to peak), Decay (time from peak to sustain level), Sustain (level while the note is held), Release (time to fade after the note ends). Every sound has an envelope — percussive sounds have fast attack/short release; pads have slow attack/long release.
In synthesis, envelope generators (EGs) shape the sound over time. In mixing, transient shapers allow independent control of attack and sustain portions of a sound — adding punch to drums without affecting the body, or softening harsh attacks without dulling the sustain.