Woot is a term used to show extreme exuberation in joyous agreement. It is best expressed by teen girls excited by an upcoming situation or just to display common happiness.
Exclamation of technology obsessed people (h4x0rZ, gamers, even txtmsg freaks) indicating feelings of excitement and satisfaction. I believe it originates from "What now, bitch!" which was shortened to "What!" and then 1337-ized to "w00t".