A return of serve is the ball hit by the player not serving to start a point. While serving is considered an advantage in a point, great returners like Andre Agassi and Novak Djokovic have been quite effective at breaking serve (winning the games served by their opponent) in their careers.
If the server is right handed, the slice serve will curve to the right side of the returner after it bounces. Therefore, the returner will likely be hitting a wide forehand shot when returning. If the server is left-handed, the ball will curve to the left of the returner, leaving them to hit a backhand shot. The key to returning the slice is to detect the spin early and start moving toward the ball before it bounces. If the server is left-handed, the direction of the spin would be reversed.
A good doubles returner tries to keep the ball away from the net player and low to the feet of the server, if the server is rushing the net, or deep crosscourt, if the server is staying back at the baseline. A deep lob over the net player can also be effective off the return.