Useful links to discover what Long Tail is:

http://www.journaldunet.com/diaporama/0610-dicoweb2/9.shtml

http://www.journaldunet.com/0703/070326-net-long-tail-definition-solution-outils-longue-traine.shtml

http://longtail.typepad.com/

http://en.wikipedia.org/wiki/The_Long_Tail

To my mind the most comprehensive explanation from the author, Chris Anderson:
http://www.thelongtail.com/about.html