You can play either as cops or racers, but both sides get ultra powerful cars and a range of power-ups to get them to the race's end. If you're a cop your job is to chase down your quarry using any means necessary, including roadblocks.
If you're playing as a racer you'll often be in a slower car than your pursuer, but you won't be tied to any of their rules and get your own powerful power-ups like the radar jammer. With its own in-game social network you don't just get to race online you can live the whole Need for Speed lifestyle with your friends and rivals.