The first thing that comes to mind when I think about 'game design' is ideas. Followed by story, characters, environments, settings, time and how the game plays. I think theres a lot involved in designing and then creating a game.
Its the job of game designers to think up games and the world they are set in and how you as the player experience it. When an idea for a game is created game designers have to think of everything about anything involved. They envision the main characters, side characters, quests, story, drive, impact, sound, style, flow and everything in between.
Games will fall into different genres which affect how the game is designed and what is most important, genres are often mixed with varying degrees of success. The game designer will usually produce an initial design document containing the concept, game play, feature list, setting and story, target audience, requirements and schedule, staff and budget estimates. Many design decisions have to be made during the development process and it is the responsibility of the designer to decide what will be implemented, based on things such as creative vision, budget or hardware limitations.
Game play is another important part of games, it is the specific way in which players interact with the game. It is the pattern defined through the game rules, the players connection to the game, challenges and how you overcome them, plot and how you connect with it. It often falls to the game designers to determine how the game is played and making sure there is consistency throughout.
Games at their basic level haven't changed much over the years, even from the time of board games. There is always a goal, always rules and always things trying to prevent you completing your goal, making it a challenge. Technology has improved making us capable of creating more and more interactive and enjoyable experiences but the fundamentals are still the same.
Using the two video games Pac-Man and F.E.A.R for example and thinking about how different, or not so, they actually are. Pac-Man is a simple top-down game based on running around a maze trying to collect all the dots on screen for points while you are chased by 'ghosts' which you have to avoid to survive. You occasionally get the power to 'eat' the ghosts while they run away from you. Music is used cleverly during game play with changes in pace and pitch to exaggerate moments of fear or excitement. The overall goal of Pac-Man is to survive as long as you can to achieve the highest score possible.
Now F.E.A.R which is a psychological horror first-person shooter. The game's story revolves around a supernatural phenomenon, which the F.E.A.R special forces team is called to contain. The player assumes the role of F.E.A.R.'s Point Man, who possesses superhuman reflexes, and must uncover the secrets of a paranormal menace in the form of a little girl. Throughout the game you are attacked by large numbers of enemies or haunted by the ghost girl and supernatural goings on. Like Pac-Man music is used to exaggerate times of excitement or fear which adds to the games immersion and enjoyability.
Most games have the same underlining fundamentals of game design, tried, tested and improved ways of making video games enjoyable, entertaining and believable. It is the job of game designers to take all these preexisting methods and make something new and unique that people will want to play and have fun doing so.