Control of the board cannot be understated. The most valuable four squares on the board are the four directly in the center of the board. In many cases the person who controls the four squares at the center of the board will have the better game and a definite advantage. It is critical to attempt to gain this control of the center.
There are two simple reasons that controlling the center of the board is important. First, a piece in the center controls more of the board than one that is somewhere else on the board. As an example, place one knight on a center square and another in one of the corners of the board. The knight in the center can move to eight different squares while the cornered knight only has two possible moves. Second, controlling the center of the board provides an avenue for your pieces to travel from one side of the board to the other. To move a piece across the board you will often have to take it through the center. If your pieces can get to the other side faster than your opponent's pieces you will often have an advantage. Being able to mount a successful attack on that side before he or she can bring over enough pieces to defend against your attack results in you having an advantage.
It is often worthwhile in the long run to be willing to exchange pieces with your opponent so that you gain the control of the center of the board. This exchange of pieces may seem wasteful at first, but pays off in the end of the game by you having a control of the tempo. Control of the center of the board also offers an excellent defensive advantage.