Over the course of the past few months, Sonic Team has been steadily working on a new Sonic game named Sonic Heroes. Unlike the Sonic Adventure series, Sonic Heroes seems to place an emphasis on action, much like the older Sonic the Hedgehog games. In the game, you can select from one of four teams--Team Chaotix, Team Rose, Team Dark, and Team Sonic. Unfortunately, we only got to see Team Sonic, which is made up of Sonic, Tails, and Knuckles.
These characters essentially follow each other throughout the entire level, so you can switch between them and make use of each character's special abilities. Tails is able to fly, so he can carry Sonic and Knuckles across great distances or reach platforms that would otherwise be inaccessible. Naturally, Sonic is the fastest of the group, and he also has a hurricane attack that he can use to defeat groups of enemies. Knuckles can glide, punch, and throw Sonic and Tails like projectiles.
There will be 14 stages in the game, each of which will pan out differently, depending on the team and path taken. There will also be different objectives that you will have to fulfill in order to successfully complete a level. There are plans to include multiplayer support as well. Sonic Heroes is scheduled for release later this year.