Originally Posted by: dvenetianFrom My understanding it's called "A 440Hz" and it's the amount of vibrations per second (I believe) that the lowest "A" note produces when in tune. The "A Octive" has twice as many vibrations (A 880Hz)

Being a bit technical the whole enchilada is this:

FREQUENCY

A PITCHED SOUND is one that has a repetitive CYCLE or PERIOD. PITCH is determined by the number of CYCLES per second and is called FREQUENCY. The term, HERTZ (Hz) refers to the mathematician, Henrich Hertz and is used to represent the number of cycles per second. The RANGE OF AUDIBLE FREQUENCY FOR THE HUMAN EAR is between approximately 16 Hz and 20,000 Hz (20 kHz). The note 'A' above Middle C (C3) has 440 cycles per second (Hz).

Check this site has some intresting facts and graphics that make it really easy to understand

http://ftp.ec.vanderbilt.edu/computermusic/musc216site/what.is.sound.html

Class, you are dismissed.. :D