Videogames are interactive; that's what separates them from other game media. As such, it is a vital component of videogame design to study and understand the player-to-game interaction.
In essence, it's really simple: the player makes a decision, communicates it to the game, and the game produces some result.
Let us call the first stage, the player making a decision, what it is: Decision Making. The second state we will call “Action.” The final stage we will call “Rewards.” So we can respecify the above as the player making a decision, acting on that decision, and receiving a reward.
Where this becomes really useful will be in my next article on the different modes of thought that go into these decisions and rewards. However, until then, there are a few minor points on this matter that need discussion.
The decision-action-reward (DAR) dynamic is a vital component of game design. Enjoyment of a game can come from any of the parts, or a combination or all of them: enjoyment of making the decision, enjoyment of carrying it out, or enjoyment of the rewards granted by it.
For example, if you're playing a puzzle game (not a Tetris-style puzzler, but a static puzzle game), figuring out the solution is part of the decision making process. Acting on that decision is often very simplistic, requiring little to no skill in doing so. The rewards of actually solving that puzzle is merely getting to solve another one that is, perhaps, more difficult or more interesting, though sometimes you are rewarded by learning more about the game system that will be tested in later puzzles. Most of such a game's enjoyment comes from solving the puzzle.
By contrast, a Tetris-style puzzle game provides some depth and enjoyment from action as well as decision making. The rewards are to continue the game and a metric for determining your level of skill compared to others or prior iterations from yourself.
Game design is, in effect, about creating a pleasing decision-action-reward loop that is self-reinforcing. That is, the enjoyment derived from executing one iteration of the loop makes the player want to execute another iteration. There are many tools in a game designer's toolkit to create the links between separate DAR elements, but we'll get to those later.
Let us define another piece of terminology: element. An element is a discreet DAR section of a game that involves a significant decision by the player and provides a significant, specific reward by the game.
An entire game is a single element, but so are sub-sections of a game. In a 2D Mario platformer, a level is an element, but so is a particular piece of a level that involves significant player decision making, acting, and provides a reward.
Looking at sections of a game's content  below a level of significance is ultimately futile; unless a basic motion of the character carries a particular significance in one of these ways, it is just not important. In a platformer, navigating a flat section with no enemies in it is not a significant piece of content.
So elements can be composed of other elements in a hierarchical relationship. Several elements are combined to make up other elements, and those elements are combined further to make up others, until at the last, all the elements are contained by the element defining the actual game itself.
The purpose of game design is, therefore, to come up with a sequence of elements that will reinforce one another, so as to keep the player entertained and keep the player playing. This sequence can be encountered in any order, whether pre-defined, player-defined, or some combination thereof. Indeed, deciding on how the combination of elements goes together is a vital part of game design.
1: I've been avoiding the term “gameplay,” and with very specific reasons. I'm attempting to take a holistic look at videogame design, one that does not seek to separate game design from graphics or create other dimensions of effort. A well-made game can use graphics as a reward (it is interaction, after all), just as it can use non-vocal sound to communicate real information.
I believe that the predilection among some of the “elite” or intellectuals in game design who attempt to speak of separating gameplay from other facets of videogame design are incorrect. That this separation is preventing them from using facets of non-“gameplay” as viable rewards as well as information feedback in decision making.