A race camshaft is a camshaft that has been especilly designed to give better performance than stock. The altered design makes for things like better lift, faster opening and closing of the valves, earlier opening, and later closing, etc. The exact effect are varied depending on the vehicle. These camshafts are generally labeled as street grind, three-quarter race camshaft or full race.
A new camshaft is a great way to help your car achieve better performance. But like most modifications, the increased speed comes with a price. In this case the price is a rougher idle and lower gas mileage.