Tactic is one of the very important aspects of tennis, and not using the correct tactic/strategy is one of the things which holds club players back from winning matches.

To make it easy for you to know what tactic is and how it is used in tennis, I first want you to answer this question: What do you want the tennis ball to do when you are playing a point? if you have never thought of what you want the ball to do then that means you usually do not use any types of strategies or tactics on the court.
Biomechanically, your racket can do 3 things to a tennis ball: 1. hit the ball high with spin. 2. hit the ball hard with power. 3. Contact the ball in different angles to get different directions on the ball, for example contact the ball early to go cross court.

As mentioned above, what you do to the tennis ball by your racket is basically start of doing tactics, the most basic tactics that players do in a rally situation are moving the opponent around in the court, you can move them across the court (right to left and left to right) or forward and backward (by doing deep and short balls). Two common tactics in Serving situation are serving out wide to create space and going for the second ball in space, or serve heavy down the middle to control time and build with inside-out Forehand (strength) to win the point.

I have to point out that the tactics which tennis players use mostly depend on what type of tennis players they are and what situation they are in. There are three types of tennis players: Aggressive baseliner like Agassi who most of the time stays at the back of the court and tries to control the rally from baseline by moving the opponent right to left or forward and backward. The other type of player is net player like Tim Henman whose main tactics are trying to get to the net and finishing the point at the net or doing serve and volley.  The third type of tennis players are consistent players who basically do not have a weapon but they know how to block every shot at you, therefore it takes a lot of hard work to win a point against of them, because they do not give you the point, they actually make you to earn it from them, I would consider Lleyton Hewitt as an excellent example of a consistent player.

The reason Agassi was known as an aggressive baseliner through out his career is because he had heavy forehands and could take the ball on raise which simply means he could dictate his tactics to his opponents from baseline, on the other hand Henman was very fast on court and had great topspin serves so his coaches trained him to be a net player who often does serve and volley because of his strengths. May be if Agassi was trained to be a net player then he would not had won all four grand slams because even he had great forehands but he was not as good as Henman in volley. So, the point is that first you have to know what type of player you are and then decide what your main tactics are going to be. So, what type of a player are you?

The other thing which I have to add is that there are three situations which a tennis player can be at: 1. attacking, 2. rallying. 3. defending. For example if you are doing a first serve and you are an aggressive baseliner and you want to attack then you can go for a wide slice serve and finish the point by a heavy forehand down the line. The other common example is if you are a consistent player and the opponent keeps on attacking and they put you in a defensive situation then you can defend by using deep slices to stay in the rally or to neutralise their shots.

Stan Orlans who is doubles specialist and has been coaching tennis for more than two decades has written some great tips and tactics about how to to play doubles:

DOUBLES TACTICS 1

DOUBLES TACTICS 2

DOUBLES TACTICS 3