Just what is a Game engine? Well a game engine is what the game is built on, it handles the physics, the rendering of the game world, sounds, animation, networking and a bunch of other things, each one being an important part of the engine.
But that doesn’t mean that it comes up all roses, game engines have to be continually updated to be compatible with todays technology, for example multi-core processors (in essence its 2 processors in one), its pretty common to have a dual core processor, but few games make an effort to take advantage of dual core, or even the recent quad core processors. Which is what the current developers are trying to do, this is because you could get different cores to do different jobs, and work together when more processing power is needed.
And since according to Moore’s law, computing technology doubles in power each year, 10 core processors are looking to be a possibility next year, so when you think about all that processing power that could be on tap, its not surprising that Game Developers are trying to make technology to take advantage of that.
And there are 2 types of game engines, subtractive and additive, subtractive is where you carve out your game world from a pre existing block, whilst additive is where you add to a seemingly infinite space. For example Valves source engine is an additive game engine, whilst the Dark engine, used by Thief and System shock 2 is subtractive. But you dont have to make a game engine, you can buy one!
And there are many advantages to buying someone elses technology, for one you dont have to spend time making it and there is usually already tutorials and a free version so that you can get to grips with it easily. But of course there are also disadvantages, in that your already spending money on something before you even get round the making it, and depending on how you purchase the technology you may even have to pay royalties on your product.