You need to be extremely good at tactics if you want to be a good chess player. Tactics is what separates a good and a bad chess player. The capacity to count combinations and variations is a must for a good chess player.

Here's an example which tells as to how one should count variations: if White begins the game with 1.e4. and Black responds with 1.d5 then with 2.exd5, White can get hold of the d5 pawn. This variation involves [1.e4 d5 2.exd5]. White gains some material here but Black captures back by 2.Qxd5 at d5. This restores the balance. This is also a variation [1.e4 d5 2.exd5 Qxd5]. There can be a number of moves in the variation.

Many variations are possible and one needs to calculate them when a position is analyzed. In the above example, Black may play 2.Nf6 instead of capturing d5 immediately. This will threaten the pawn and the knight. This is done so that capturing is possible later on. To protect the pawn if White plays 3.Nc3, Black may equalize with 3.Nxd5, ready to meet 4.Nxd5 with 4.Qxd5. however if white moves 3.c4 instead of 3.Nc3 to support the pawn then Black cannot capture d5 pawn as White will capture the pawn and the knight finally. Black will have 2 pawns. Black as it looks will lose the game irrespective of how he plays in the future. Of course the tables can change if White makes a big blunder!

The combination is not dependent on the reply of the opponent. To be valid, the combination has to win anyhow. It is a combination that puts on something. A benefit has to be made by all the variations that may arise after the 1st move. When you count combinations and variations, keep into account the best reply of the opponent.

To display a short game, let us take an example:
1. [e4 e5]    2. [Bc4 Nc6]
3. [Qf3 d6] 4. [Qf7#]

The symbol "#" represents checkmate. Black is blundering checkmate and there are no options available here. Black could have protected itself by 3.Nf6 if it had foreseen White's threat on f7. The move 3.Qf3 was not a very good move by White, and Black could have been in a better position had it played sensibly.

Here's another example, this one uses a combination:
1. [e4 e5]                  2. [Nf3 d6]
3. [Bc4 Bg4]            4. [Nc3 h6?]
5. [Nxe5! Bxd1??]   6. [Bxf7+ Ke7]
7. [Nd5#]
A double question mark after the move means a blunder whereas a single question mark represents a mistake. A very good move is denoted by an exclamation mark and # represents checkmate.
The number 4 move is not a good move; in fact it's a mistake because it allows White to use the winning combination which begins with 5.Nxe5! White threatens the Bishop on g4 and at the same time, it also threatens to checkmate by 6.Bxf7+ and 7.Nd5. Black could have protected itself with 5.Be6 but it made a big blunder with 5.Bxd1?? and this allowed White to checkmate. This is known as Phillidor's Defence.

The next example is called Russian game; here Black completely missed an important move made by White.
1. [e4 e5]              2. [Nf3 Nf6]
3. [Nxe5 Nxe4?]   4. [Qe2 Nf6??]
5. [Nc6+]
The third move is not a good move because according to the opening theory 3.6d should be the first choice. The 4th move allows White to do revealed check. Black's queen is threatened by the white knight at c6. Black is also in check. He is bound to lose the queen as well as the game. Black could have avoided the loss if he had used 4.Qe7. Now, if White captured Ne4 with 5.Qxe4, Black could have replied with 5.d6 and captured it back. After the move of [6.d4 dxe5 7.Qxe5] White has an edge due to the extra pawn.

To be brilliant in chess, one needs to be accurate with the number of variations. There are a number of possibilities so it is not always easy to count the variations. But only certain moves are important in each position. One can rule out the rest of the moves. To select moves quickly, you need to have experience. If you really want to improve your tactics, you must play against computer.