   which parts are being ported
  I wonder which parts of a game are being ported for different consoles or operating systems? For example, do graphics parts be ported?sorry for my english..   
  Any parts that are built on top of non-portable APIs needs to be ported.So yes, the graphics APIs on most systems are different, so that system has to be re-written (ported) for most systems.You can avoid this by using a game-engine that has already been ported to all the platforms you want to use. The game engine acts as a "portability layer", which contains all of the platform-specific details. You can then write code that doesn't rely on any platform-specific features (you instead use the game-engine's versions of those features).   