This was the last (2D Platform) Sonic game to grace the Sega Genesis. It is the conclusion to the story that began in Sonic 3. This is the Sonic playthrough. When playing as Knuckles you have alternate abilities and paths through the levels, so I will very likely get a Knuckles version up at some time.
In addition to being a full stand-alone game, you can also pair this cartridge with other Sonic games for the Genesis to add additional features to them. For instance, you can play as Knuckles in Sonic 2, which I will also probably get on here at some time.