What is a sound envelope? What does ADSR stand for?

A sound envelope is the (ADSR) Attack, Decay, Sustain and release of a sound. This is the way how sound changes volume over time. Attack is the loudest part of ADSR, as from this point the sound decreases, to decay, then sustains the sound for a short time (depending on what has been used to make sound e.g. plucking would create a short sustained sound) and release, where the sound finally comes to an end.

Examples of sounds with long and short attacks with a long and short sustain - 

• Violin being bowed - Long attack, long sustained. Short attack sound can be created by the violin being plucking which is a short sustained
• Piano key held down until release - long attack, long sustain lingering on the key. Depending on force (faster tap on key will be a shorter, sharper sound) which is a short attack sound with a short sustain
• Revving of a sports car - Long attack, short sustain - release after attack
• Dry leaf being stepped on (crunch) one solid sound with short attack, short sustain
• Symbol - Short attack, long sustain 

Examples of where these sounds could be used - 

• Violin with long attack and long sustain could be used in an emotional scene of a film, creating a mood in which we can detect what is happening within the scene. Plucked violin sound (short attack, show sustain) quick, sharp plucks which could be used in Psycho shower scene
• Piano key with long attack and long sustain (lingering key) could be used in a horror film (lower key), as it is a slow sound which can build up creating a overwhelming climax in a killing scene for instance Scream. A higher pitch key or key used with less force could be used in a thriller film as it sounds similar to a slasher sound, sharp and straight to the point
• Revving of the sports car which is a long attack with a short sustain could be used within a racing  film such as Need For Speed or another car based action film
• Dry leaf being stepped on with short attack, short sustain; could be used as an atmospheric noise in any type of film - mood, autumn, nature etc. 
• Symbol with short attack and long sustain, karate films