The Importance of Seat Position

So you are sitting at a $3/$6 Hold'em table, and the cards are dealt. You decide that with your hand, it is worth $3 to see the flop. You call. The next two players fold, the following player raises. Now you decide that it isn't really worth $6 to see the flop, and you wish you had never placed a bet in the first place.
That is difference table position makes. Had you already known someone was going to raise, you would've saved three dollars that hand by simply folding. Knowing what your opponents are going to do makes a difference.
A full Hold'em table is broken into three pieces. The first three players are said to be in Early Position. They must act before most of their opponents. Because they are acting first, players in early position should avoid playing marginal hands. After all, there are up to nine more hands at the table to contend with. Instead, players in early position should limit themselves to playing only premium hands.
The next set of players at the table are said to be in Middle Position. They have seen some of their opponents act, so they have an idea of what they are up against. If no one has raised, players in middle position do not need to limit themselves to only the premium starting hands. Another advantage of middle position is that there are fewer players who have not acted, and so the chances that someone will raise you are lower.
The last set of players are said to be in Late Position. They have the advantage of having seen most of their opponents already act. The very last player to act has the most advantage that table position can offer. They have seen everyone act, and should they decide to call a bet there is no chance of someone raising them. Players in late position can play a wider range of starting hands because they already know how many opponents they will be facing.
The general guide is to play very tightly in Early Position sticking to only the best starting hands. In Middle Position, stick to playing good starting hands, but they need not be the very best. In Late Position players can afford to play much more loosely.
Of course there are different betting strategies that can be used depending on your table postion and the actions of your opponents, but the first thing to understand is that where you are seated in relation to the dealer makes a difference. You can improve your game right away by simply paying attention to your table position.