Neo Sonic Universe is actually two games in one: it includes a "normal" mode, similar to the classic Sonic games, in which you collect rings, kill badniks, defeat bosses and clear stages; and a battle mode, in which you fight against another character (human or CPU).
The normal mode features different levels with a boss to defeat upon level completion. You can play as any one of 5 characters: Sonic, Tails, Knuckles, Amy, or Shadow. Each has its own special moves, but you must beat the game as Sonic first before the other characters are unlocked. The controls are simple--use the arrow keys to move and use CTRL to jump. There are few good Sonic remakes online, but this is definitely one of them. It stays true to the original and is void of any big glitches that most other Sonic games are riddled with.
The battle mode is a 2D fighting game with different levels to play on, each with its own special effects. You can play an arcade battle, in which you battle against each character and clear levels, or a VS game, in which you fight against the computer or another player. A big plus is that the controls are configurable and you can also use a joystick in this mode. There are two bars for your player--one shows HP and the other shows Mana Points. Mana points are used for for magical attacks, which are activated by using a specific sequence of keys, and are different for each character. The battle mode is an excellent addition to this game and will not disappoint.
There are many Sonic games available on the web, but few come close to the quality that this remake exhibits.