Having 'perfect pitch' is when someone is able to sing any requested note that is named (by someone else) arbitrarily in thin air, and without another note needed to be played first, while 'relative pitch' is when someone is able to sing any requested note, after an inital note (such as C) is played first.

Having 'perfect pitch' is a very rare gift (or curse to many). You are either born with it or not. It cannot be learned or developed. Here's a link to some informative articles re: Perfect (absolute) vs Relative Pitch:
http://www.amarilli.co.uk/piano/perfectp.asp
http://courses.dce.harvard.edu/~musie139/PerfectPitch.html
http://www.edwardwillett.com/Columns/perfectpitch.htm

Here are a few famous musicians/singers who possess perfect pitch:

Julie Andrews
Ludwig van Beethoven
Mariah Carey
Nat King Cole
Bing Crosby
Ella Fitzgerald
Yo-Yo Ma
Wolfgang Mozart
Oscar Peterson
Frank Sinatra
Barbara Streisand
Stevie Wonder
Brian Wilson (Beachboys)


BTW (just in case you're wondering) I myself don't possess 'perfect pitch' , but have developed good 'relative pitch' recognition thru interval ear training. Curious who on this forum actually has 'perfect pitch'.

Scott