Key signatures

[nuroute]

I've heard of key signature before, but could someone tell me whats the deal on them ,what are they or even where can i find a good article about them.

[Copenhagen]

The internet is a good place to start...found the following sites that I haven't read up close...but they might help answer your question.

http://www.myriad-online.com/docs/manual/english/key.htm
http://ourworld.compuserve.com/homepages/Neil_Hawes/theory11.htm
http://cnx.rice.edu/content/m10881/latest/
http://www.freevoicelessons.com/february_19,_2001.htm
http://www.slowplayers.org/SCTLS/modes.htm
http://web1.hamilton.edu/javamusic/KeySigDrill.html

Let me know if they were any good... ;)

[PHILharmonic]

Attention, class is in session...lol
back to the Grove Dictionary of Music & Musicians:
In Western notation the group of sharp or flat signs placed at the beginning of a composition, immediately after the clef, or in the course of a composition generally after a double bar. The signs affect all notes of the same names as the degrees on which they stand, and thus define the key of the composition.

Western notation basically refers to notation on basic 5 line staff.