To give you some input from a dev standpoint. Android needs two core res files to run its gui. Framework-res.apk, and SystemUI.apk. What samsung did was theme the SystemUI apk and provide a TWFramework-res apk. Basically they rearranged the way the gui is displayed to use their TWFramework-res apk instead of the stock one.
Of course there's little extra apps that make up the tw experience as well. I mean OEMs usually theme all of the system apps, the same thing rom developers do to make their rom standout among the crowd. Plus the extra little exclusive features like multiwindow.
Sprint GS3 Running TN's Msg and Chubbs