Game Engines

The game engine is the software that actually makes a game work. Without it, you would just have a load of random data not doing much at all. Game engines are designed to work on a variety of platforms. For example, Valve's Source engine works on the PC, Playstation 3 and xbox 360, even though it is about 50 years old :P Still, Team Fortress turned out nicely, except for crappy hit detection and many many bugs.
Anyway, a game engine consists of several smaller engines. These include a rendering engine, physics/collision engine, sound, animation, AI and more. The actual term 'game engine' was used when the release of Doom and Quake appeared. From these, many developers brought licenses to use these engines and then simply design their own visual style, clearly saving them time and money.
Subtractive and additive are terms used for difference level creation processes. The additive process is where the designer must create the level in a void which is endless and empty. If this process is used a skybox is required to seal off the map. A map must have no leaks, otherwise this can cause lighting issues and textures becoming distorted and blurred. If you think about a fish tank, if there is a hole then the water will leak out, the same applies for a level, but it needs a roof of course. Subtractive on the other hand is where there is no void, just an infinite solid which must be carved into. This method means that there is no chance of leaks but it isn’t as easy to manipulate as the additive process. Maps such as Counterstrike and Team Fortress 2 are created with the additive process, in a program called hammer. It is a free program to download off Steam, in the Source Developer Kit.
As for next generation engines, they must have a long life span and to be adaptable for many games. This will help reduce costs, as developing a game can cost millions. Only the highly popular games will easily provide a profitable income.