That would be disfluency: it's the formal linguistic word for the um's and uh's that we have in our day to day speech. "You know" and "like" are other common American disfluencies (different cultures have different disfluencies). There's actually a recent Saturday Night Live skit which was basically a guy who could only speak in disfluencies. Funnier than it sounds :-)
Via Powerset, which was via Wired.
p.s. Also, ummmm... never mind, whatever.