There is a great deal of terminology in the chess community that is unique to players. A lot of it is unnecessarily insulating, a way to try to separate players from others, to create an in-crowd. As much as possible, we will avoid using jargon.
There are, however, some ideas that have currency. The three stages of all chess games is one of them. Each game has an opening, a middle game, and an endgame. These are studied extensively and books are often written on just one of these three aspects of the game.
Most beginners are encouraged to spend their time mastering established chess openings, and this is a good idea for those who naturally have an appetite for repetitive learning. Others, though, may want to gain an appreciation for why the openings work in the first place. If that's your personality type, don't worry about fooling around a little bit before you start memorizing openings. Because there is so much free chess software out there and a large online community of chess players, you can play openings lots and learn a lot about what works and what doesn't through trial and error.
The middle game is the hardest phase for which to enumerate specific rules of play that work over and over again. The best approach to take varies, and in this way it is the part of the game that is most like an actual battle. What you will learn over time are tactics and strategies that are effective in different combination based on your particular opponent, the conditions on the board, and other factors you can't anticipate until each game begins. Tactics are the particular actions you take, while strategies are overall approaches. For example, you might decide that your strategy is to throw an opponent off, so your tactic might be some random moves with a knight. (This is just for the sake of illustration; it actual sounds like a terrible tactic.)
The endgame is where all of your experience over time will really pay off, because it is a real test of patience and benefits from a deep understanding of the role and capacity of each of the pieces. We will not discuss endgame strategies much in this version of this introduction, but, as readers seem to be looking for it, we can add some thoughts on the endgame. Don't forget that you can give us feedback on every page through the comments at the bottom - the more of a clear need for discussing the endgame we see, the more we will discuss it.