How does snort do packet signature detection?

["Chhabria, Kavita - Apogent" <kchhabria () apogent com>]

Hello all:
 
Can somebody please explain me in simple words, what is a packet signature
or rather what is contained within a packet signature and why is it useful.
How does an IDS detect an attack based on packet signatures?  What
information is the IDS looking for within a packet, when it says that is
doing packet signature inspection?
 
Thanks a lot,
 
Kavita Chhabria
Systems Developer
Apogent Technologies
(269) 544-7515
 <mailto:kchhabria () apogent com> kchhabria () apogent com