Sonic x Shadow Generations is a remake of Sonic Generations. With it, SEGA has added a bunch of new modes, content, and improvements to the original game, as well as some more subtle changes.
It’s hard to tell what has been added or taken away from the original or changed if you don’t pay attention to some specific activites, cutscenes, and other things. Most of the changes are actually pretty small outside of the big new mode and a few additional collectibles to find in levels.