Top-spin is the most common type of spin seen on courts today; the majority of ground shots played at higher level games will have some degree of top-spin. It is used to keep the ball low over the net whilst hitting with power, and gives the ball an extra ‘kick’ as it bounces, making for an uncomfortable return shot for your opponent.
It is achieved by whipping up the back of the ball, hitting from low to high. Your racquet face should be more or less parallel to the court as the stroke finishes, whereas it would be perpendicular if there was no spin. The racquet rolls over the top of the ball, causing the ball to spin through the air in the direction it is travelling.
It is called top-spin because the spin comes from friction with the racquet at the top of the ball. When playing a top-spin shot, you are effectively reversing the direction that the ball is spinning in, so greater energy is used in topspin than in hitting flat (ie, with no spin). Top-spin is used as a way of controlling powerful strokes; if you hit a ball flat with the same amount of power, it would have no force to make it dip over the net, and would just rocket out of the court.
Sergi Brugera, a former professional tennis player, was able to hit a tennis ball with top-spin of 3300 rpm, meaning the ball would turn 70-80 times between the time Brugera played the stroke and his opponent received it.
They key element to a successful return of a top-spin shot is your positioning on the court. You should be aware that a top-spin shot will bounce much higher than a flat shot, and it will also dip sooner. As a general rule, the greater the arc of the ball, the greater amount of top-spin is on the shot. If you are able to step up to the ball and take it while it is rising off the bounce, you may be able to hit an aggressive return. If you take the easier option and wait for the ball to reach the top of the arc, you will have been forced further towards the back of the court, giving your opponent more time to recover their position.