TCP has a random seq number that need exist in each of it's packet. Each OS has it's own way to come up with this "Random" number. this leave us space to Detect . "fingerPrinting" which OS is  packets been sent from.

Ref: http://en.wikipedia.org/wiki/TCP/IP_stack_fingerprinting

http://nmap.org/book/osdetect-methods.html

Tools: Nmap