They are called "fillers," "speech fillers," or "verbal fillers." Here are a few examples:

http://www.ag.iastate.edu/aginfo/speech_delivery.html

"Don't use verbal fillers such as "um," "uh" and "you know." Instead, pause silently."


http://people.hsc.edu/faculty-staff/cdeal/students/rehear.ht...

"Disadvantages include a lack of organization, increased anxiety, insertion of verbal fillers such as "um," "you know," and the inability to include prepared visual aids such as graphs and charts."


http://home.bluemarble.net/~langmin/convers.htm

"The speaker then keeps the turn for a time by means of verbal fillers: I mean, well, sort of, anyway, you know. (Haven't you had the experience of getting exasperated when someone seems to overuse the latter? Just remember next time that this person is really sending regular signals that s/he needs time to organize and isn't finished yet). You might feel the need to stall for time with words such as um, actually, let me see."

Fuad

FuadMYahya is correct, but the terms offered break down a little further, into the self-explanatory "filler words" (bueno, pues), "filler phrases" (you know) and "hesitation noises/sounds" (um, uh). There are other terms as well, but these are probably the most familiar.