A topspinning cricket ball behaves similarly to top spin shots in tennis or table tennis. The forward spinning motion impedes air travelling over the ball, but assists air travelling underneath. The difference in air pressure above and underneath the ball (described as the Magnus effect) acts as a downward force, meaning that the ball falls earlier and faster than normal.
In cricketing terms, this means that the ball drops shorter, falls faster and bounces higher than might otherwise be anticipated by the batsman. These properties are summed up in cricketing terms as a "looping" or "loopy" delivery. Also, the ball travels straight on, as compared to a wrist spin delivery that breaks to the left or right on impact. A batsman may easily be deceived by the ball, particularly given that the action is quite similar to a leg break.
In delivery, it is gripped like a normal leg spinner. At release, the back of the hand faces to the cover region and the palm of the hand faces the mid wicket region. The ball is then released over the third finger with the seam going straight on to the batsman. It should be noted that leg spin balls are frequently imparted with both top spin and side spin, a ball presenting with roughly equal amounts of both is usually called an "overspinning" leg break.
Tactically, a bowler will bowl topspinners to draw a batsman forward before using the dip and extra bounce to deceive them. In particular, batsmen looking to sweep or drive are vulnerable as the bounce can defeat them.