Hello,
quick explanations :
* b1 = normal strike
the basic strike, without effect
* b2 = safe strike
more soft strike, with a more round trajectory
* b1+b2 = acceleration
strong strike, with a little bit of top spin effect, to hit the ball as strongly as possible
* b1 + up = short ball
to send the ball around mid-court, allow to do more crossed shots
* b2 + up = slice
Backspin effect, slightly on side, it's a defensive strike that will give a low rebound to the ball, but it can be used to go on net as well, because the ball is slower and it gives you more time to rush to the net
* b1 + down = top spin
Top spin effect, that will give a high rebound to the ball, good to push your opponent more far of his baseline
* b2 + down = defensive lob
very high ball trajectory, but a bit slow, used when your opponent is at the net, cannot be a winner shot in most of time
* b1+b2+up = drop shot
very short trajectory, falling just after the net
* b1+b2+down = aggressive lob
high ball trajectory, but still a bit fast, used when your opponent is at the net, can be winner shot sometimes
you can find more information here :
http://en.wikipedia.org/wiki/Category:Tennis_shots
and here :
http://www.answers.com/topic/tennis-shots
I hope it'll help !