One of the charms of the game of chess is the interplay between tactics and strategy. Tactics refers to "tricks" or "combinations" that achieve material advantage or checkmate in a couple of moves, strategy refers to long-term planning and the proper placement of the pieces on the board in the absense of any short-term opportunities.
In describing tactics and strategy, we will be using the algebraic notation for squares on the chess board.
Values of the pieces
Since one objective of tactics is to obtain material advantage, one first needs to understand the values of the pieces. A knight is about as valuable as a bishop, but less so than a rook. Three pawns will overpower a knight in the endgame, but in the middlegame a knight is often more powerful. A knight and a bishop are stronger than a single rook. Two rooks are stronger than a queen, but not by much. One commonly used simple scoring system is 1 point for a pawn, 3 for a knight or bishop, 5 for a rook, and 9 for a queen. Under a system like this, giving up a knight or bishop in order to win a rook ("winning the exchange") is advantageous and values about two pawns.
A fork is a move that uses one piece to attacks two of the opponent's pieces at the same time, thereby achieving material advantage. Knights are often used for forks: they jump to a position from where they attack two pieces. A quite common situation is a white knight jumping to c7, thereby threating both the rook at a8 and the king at e8. Pawns can also fork enemy pieces: by moving a pawn forward, it may attack two pieces: one diagonally to the left and one diagonally to the right. A common situation is the move Pawn d2-d4 forking a black bishop at c5 and a black knight at e5.
A queen move also often attacks two pieces at the same time, but this is only useful if both pieces are undefended.
A pin is a move which forces one of the opponent's pieces to stay put because moving it would expose a more valuable piece behind it. Bishops and rooks can pin other pieces. A pin that often occurs in openings is the move Bishop b5 which pins the knight on c6, because moving the knight would expose the king on e8 to a check. A common way to win the queen is to pin her to the king with a rook, for instance with a white rook on e1, the black queen on e7 and the black king on e8.
The German Zwischenzug means "intermediate move"; it is a common tactic that occurs in almost every game: instead of countering a direct threat, which the opponent expects, a move is played which poses an even more devastating threat, usually an attack against the queen or the king. The opponent has to counter that threat first, and this will ideally change the situation to his disadvantage.
When you plan your tactics, you should always watch out for a Zwischenzug. Don't assume that the opponent has to counter your threats immediately. It is a good practice to always check whether your opponent has a check or a move that threatens your queen. Conversely, anticipate your opponents threats and plan a surprising Zwischenzug.
Often it is necessary to throw the opponent's position out of balance by first sacrificing some material, to be regained with interest a couple of moves later. Pawn sacrifices in the opening are known as gambits; they are usually not intended for material short-term gain but instead to achieve a more active position.
Direct attacks against the enemy king are often started by sacrifices; a common example is a bishop sacrificing itself on h7, checking the king on g8 who has to take the bishop, after which the white queen and knight develop a fulminant attack.
Attacks against the king
Attacks against the castled king are usually justified by some imbalance: you have more firepower on the king's side than your opponent, or the opponent weakened his king's position by moving one of the pawns in front of the king.
Many mating attacks are introduced by sacrifices: if mate is the goal, material doesn't matter anymore. The queen is almost always the most important piece in a mating attack, since she has various ways of mating a king. The most common of which is a direct "contact check" while being protected by one of her own pieces, for instance white knight g5, black king on g8 and the queen mates at h7, or white bishop at f6 or h6 and the white queen on g7 mates the black king on g8.
Don't assume that every move in a mating attack has to be a check. Often, a check just drives the king to a better position, or weakens your own setup. Try to find "quiet" moves which seal the deal.
For beginners, it is not helpful to memorize opening moves; instead, by following a handful of principles, one can quite easily achieve a decent position for the middle game.
The most important part of the board is the center (e4, d4, e5, d5). It is important to place pawns in the center or to control it in some other way. Another major goal of the opening is to move the king away from the dangerous center and achieve castling. Every move should contribute to these goals and one should avoid losing time by making useless moves such as h7-h6. The white knights are usually developed to c3 and f3. The queen should avoid moving too early and too far into enemy territory, because otherwise the opponent will be able to gain time by playing developing moves which at the same time threaten the queen. Once castling has been achieved, the remaining bishops and knights should be developed so that the rooks on the first row become connected and can operate more effectively. This usually ends the opening phase of the game.
All other things being equal, the side which controls more space on the board has an advantage. More space translates into more options, which can be exploited both tactically and strategically. So if all your pieces are developed and you don't see any tactical tricks, try to find a move which will enlarge your influence, particularly in the center.
Knights are easily chased away with pawn moves. Therefore it is important to spot "holes" in the enemy position where a knight cannot be attacked, because the pawns have already moved past. Once such a hole is identified, a knight should be maneuvered to that location. An unchallengable knight on the fifth row is a strong asset, and a supported knight on the sixth row usually decides the game.
Unless there is a good reason for it, knights shouldn't be placed at the borders (and never in the corners) of the board, because there they control far less squares and can often be captured.
Pawns are most powerful if they come in groups on contiguous files. Isolated pawns, those without pawns on adjacent files, are often weak and also provide a nice spot for an enemy knight ahead of them. If your opponent has an isolated pawn, first try to block it by placing a piece ahead of it, and then attack it with rooks. The same should be done with opponent's pawns that were "left behind", meaning that the pawns on the neighboring files have already advanced.
Two pawns of the same color on the same file are called double pawns; they are weak, especially so if they are also isolated, because they cannot protect each other and because they hinder each other's advancement.
In the endgame, "passed pawns", those which cannot be hindered by enemy pawns from promotion, are strong, especially if they are advanced. A passed pawn on the sixth row is roughly as strong as a knight or bishop and will often decide the game.
A bishop always stays on squares the color it was born on. This is not a big concern if you still have both bishops, but once one of them is gone, you should keep in mind that you now have a hard time attacking or defending squares of the wrong color. If you have only one bishop left, you typically want to move your pawns to squares of the other color so that they don't block the bishop and so that the enemy pawns are stuck on the right color and can be attacked.
If you don't see a good square for development of a bishop, you can consider a fianchetto: pawn g2-g3 and bishop f1-g2. This forms a strong defense for the castled king on g1 and the bishop can often exert pressure on the long diagonal h1-a8. After a fianchetto, you should not give up the bishop too easily, because then the holes around the king can easily prove fatal.
To decide whether in a given position a knight or a bishop is more powerful, several aspects have to be taken into account: if the game is "closed" with lots of interlocked pawn formations, the knight will be stronger, because it can hop over the pawns while the bishop is blocked by them. A bishop is also weak if it is permanently blocked by his own pawns, which are arrested on the wrong color. In an open game with action on both sides of the board, the bishop will be stronger because of its long range. This is especially true in the endgame, if passed pawns race on opposite sides of the board: the bishop will always win over the knight here.
An endgame in which both parties have bishops living on different colors is almost always drawn, even if one side is two pawns ahead.
Rooks are most powerful on half-open files, i.e. files which don't contain pawns of your own color. They are also useful on open files without any pawns in order to penetrate into enemy territory (most likely to the seventh row).
In the endgame, if you have a passed pawn which is a candidate for promotion, the rook belongs behind the pawn to support its advance.
During the middle game, the king mostly stays in a corner behind his pawns. Moving these pawns should be avoided because that weakens the king's position. However, as the rooks leave the first row, there is a danger of an enemy rook invading the first row and mating the king, so sometimes it is necessary to move one of the pawns in front of the king to counter these mate threats.
In the endgame, the king becomes a strong piece. With reduced material, mate is not an immediate concern anymore, and the king should be moved towards the center of the board.
In general, it is a good idea to defend your pieces, even if they are not currently attacked. This way, many tactical tricks of the opponent won't work. Conversely, if you spot undefended pieces of the opponent, you should think about exploiting the situation with a tactical combination.
- John Nunn: Understanding Chess move by move, Gambit 2001. A top players explains the thinking behind every single move of several master class games.
- Jeremy Silman: The Amateur's Mind: Turning Chess Misconceptions into Chess Mastery, Siles Press 1999. A chess teacher analyzes and corrects the thinking of advanced beginners.