Information from Wikipedia:

ADSR envelope

ADSR

Schematic of ADSR


Attack time is the time taken for initial run-up of level from nil to peak.
Decay time is the time taken for the subsequent run down from the attack level to the designated sustain level.
Sustain level is the amplitude of the sound during the main sequence of its duration.
Release time is the time taken for the sound to decay from the sustain level to zero after the key is released.


Excerpt above from: http://en.wikipedia.org/wiki/ADSR_envelope#ADSR_envelope (accessed 10 April 2010)


ADSR by Frank Chen
http://cnx.org/content/m19007/latest/


Sample sounds from http://www.freesound.org/:

Piano:


Techsetsu__Grandmither_s_Piano_14_.wav
File added by Techsetsu on Dec 2, 2005

piano

http://en.wikipedia.org/wiki/Piano


Guitar:


SpeedY__Clean_A_str_pickFile added by SpeedY on Oct 16, 2005

guitar

http://en.wikipedia.org/wiki/Guitar



Drum:


TFUZ13x3_SM7B_VELO10.wavFile added by sandyrb on Mar 18, 2010

drum

http://en.wikipedia.org/wiki/Snare_drum

Violin:


violin_bit_5.aif
File added by sofie on Apr 20, 2005

violin


http://en.wikipedia.org/wiki/Violin

It is important to understand the attack and decay of the
instrument on the track if you are going to properly
apply dynamics signal processing (gates or compression).