how can he do that??!!

can someone tell me how does steve vai repeat his playing with a delay, i mean,
he play's a lick, then it's playing on the backround, so he play's over it, you know?

It's called sampling or looping... There is a floor pedal that he steps on... which starts recording... then he plays whatever part he wants to play.. then hits stop.... Then the loop pedal repeats it in the background... The DigiTech JamMan is a good loop pedal in which you can doo tons of stuff. Some artists like Keller Williams use multiple instruments to loop.... he will play a drum line... loop it... then go over and pick up the bass and play a bass line and loop that over the drums... then play a rhythm track and loop that... and then sing and play lead over it. It's pretty cool to watch.

I love my Jam man.

As far as I know there is also a function on the Line6 Dl4 Delay unit that allows you to have a phrase delayed, and the subsequent one dry, whilst the other still repeats...if you get what I'm saying...but it certainly doesnt get the same effect as a phrase sampler.

ohhh.... i know looping but i got confused because someone talked about delay

Well Vai does it by looping... Some delay pedals do have a feature to sample a few seconds of sound and loop it... But it's not an extensive feature like a full blown loop pedal.

I saw a video recently of a "live" performance of KT Tunstall "black horse cherry tree"

It was on VH1. She had a pedal that did looping. I think during one part of the song she had about 4-5 diferent things going on. It was a mixture of vocal and gtr that to me was very impressive.