If you have played chess at all, you know it is easy for the two sides to trade pieces: your knight takes my bishop, my pawn takes your knight, and we're even. But if your knight takes my bishop and I can’t capture your knight, you gain an edge that probably will be decisive. If one player has more pieces than the other, he usually wins without much trouble; between good players, a one-piece advantage is enough to cause the disadvantaged party to resign. Thus the most important moments in a chess game generally occur when you take one of your opponent’s pieces and he gets nothing back, or vice versa.
So how does it happen that one side takes another side’s pieces for free? Between beginners the common answer is that you wait for your opponent to blunder by leaving a piece unguarded, then you just take it―and hope you aren’t the one to blunder first. Chess games that go this way aren't terribly interesting, and they make it hard to understand what all the fuss over the game is about.
The fuss arises because there are moves you can make that force your opponent to cough up pieces unexpectedly. All his men look safe; but then you play a knight fork, a move in which your knight attacks two of his pieces at once. He only has time enough (one turn) to move or protect one of them, so you take the other for free. It's all very satisfying; and it's even better when you first capture his bishop, and he recaptures; then you check his king, and it moves; and then you play the knight fork, winning a piece. What makes this so pleasing is that you've planned the fork and forced your opponent to step into it by playing a few initial moves that forced his replies. These sequences―the little clusters of moves that win your opponent’s pieces―are known, again, as tactics. A tactical sequence generally is a short bunch of moves that wins material (pieces or pawns) or that forces checkmate. Such a sequence also is known as a combination. (Some people quarrel over the distinction between tactics and combinations. We won't.)
Now there also are other types of moves you can make in chess that aren't meant to win any pieces. Indeed, during a game you often will have no way to play one of those nifty tactical sequences, so you instead try to improve your position: you put your pieces onto squares where they have more room to move or are aimed at a part of the board where you are trying to put together an attack; or you move your pieces around to fend off your opponent’s attempts to launch attacks of his own. This sort of play is called strategic. You are working toward general, long-term goals, and perhaps laying the groundwork for a tactical strike of the sort described a moment ago. When you make these sorts of moves you may well not be seeing many moves ahead. You just are arranging your pieces the way you like, and your opponent is doing the same. Since you aren’t making any immediate threats, your opponent is free to go about his business in ways that may be hard for you to predict.
Strategy and tactics both are important, but tactics are more important. If you're a whiz at finding clever moves that take your opponent’s pieces, you will be a terrifying opponent, have a good time playing chess, and win lots of games regardless of whether you know a great deal about strategy. If you're a whiz at strategy but not much good at tactics, you will have trouble winning or having fun because your pieces will keep getting taken. You certainly want to know something of strategy; you need ideas about what you can do with your pieces that will create eventual tactical opportunities for them. We will talk about it along the way. The point is just comparative: if you want satisfaction, you had best start by learning how to play tactics―how to spot and execute sequences of moves that allow you to take your opponent’s pieces.
What was said about strategy can be said as well about openings. You can spend enormous time mastering the details of an opening―say, the Italian Game or the French Defense. The yield of those efforts, in victories and in fun, probably will be small. You frequently will find that your opponent’s play drags you away from the opening you studied; and even if not, the payoff of a successful opening usually is a minor advantage in position. By itself the advantage will not win you anything or bring you much pleasure. What will bring you immense pleasure, whether or not you know much about openings, is taking your opponent’s pieces. And to do that you need to learn how to use tactics―the weaponry of the chessboard.
All this advice assumes you are not a strong player already. Once the material on this site is behind you, close study of openings and subtler points of strategy will make better sense. One false move in the opening and your goose is cooked if you are playing Garry Kasparov; but this is a site mostly for novices, so if you are reading it you probably should not be planning to play Kasparov anytime soon. You should be planning to play others of at least roughly your own strength―probably friends who are casual players, or opponents at the local chess club or on the internet. If you keep playing you will move on to better players, but it still will be a long while before a deep study of openings really pays off. In the meantime all of your opponents―even the strong ones―will give you plenty of tactical opportunities; they will commit oversights that allow you to play pretty combinations and win pieces if you are sharp enough to see the chances for them. Acquiring this sharpness has nothing to do with memorization. It's a skill you gain by learning what clues signal that a combination may be possible, and by studying how to turn those clues into ideas that work.