So everyone knows about games and what they look like and how they feel but what are the building blocks of a game? Instead of reverting to raw code every time a developer decides to create a game they will use a starting point called a game engine. This is what allows physics, audio, graphics and rendering and a whole host of other aspects. The developer will start with this basic core structure and build on it until it is a diverse and original game. Games can all use the same engine and still be amazingly unique, its like the engine is the canvas and paints for an artist and the artist then using the same materials creates a range of completely different masterpieces. Now, not all developers own their own engines and they are faced with the option of constructing their own or buying the rights to another pre-made one. An advantage of buying the rights is that it is ready and waiting to get building on and it is hopefully reliable and accurate. A plus side of making your own, if you have the ability to would be that you could create something with totally new and unique properties, new more realistic physics and graphic quality, (Geo-Mod engine for Red Faction) but then of course you have to make it and this is time consuming.
There are two ways the games can be made from these engines, additive and subtractive. An additive design is one where the developer starts with nothing and builds in a sky box, slowly filling in the world from within the sky box. Engines like the Half life and Quake engines all use the additive method. The subtractive method begins with the opposite of the additive, a infinite solid which the user begins to hollow out into the world, this will reduce the chance of leaks (empty space outside the game being visible) but is a little trickier to handle. Game engines that use this method are ones like GeoMod and Unreal.