Motion blur can be used as an artistic technique to exaggerate or enhance an image, but if it's not used properly it can ruin a photograph. Think about it: You can probably tell the difference between a photo with "cool" motion blur and one that just seems blurry. The majority of sports photography relies on high shutter speeds to freeze a moment in time and create a clear, dramatic image.
Higher shutter speeds limit the camera sensor's exposure time, narrowing the period of time in which an image is created. This is useful for minimizing camera shake introduced by the photographer's hands, but more importantly, it minimizes the visible impact of anything moving within the frame. Considering the speeds at which most sports move, that's pretty important! Shutter speeds of 1/1000 second, 1/2000 second and up can freeze fast-moving subjects to create sharp, in-focus images. By contrast, a slow shutter speed that creates motion blur would be something like 1/15 of a second -- much, much slower.
Shutter speed is related to both ISO speed and aperture size; it's important to know that a high ISO setting increases the camera sensor's sensitivity to light. High ISO makes it easier to shoot in low light conditions, but very high ISO can also introduce unwanted grain into a picture. A high ISO setting is necessary for extremely fast shutter speeds, since the camera must be able to take in tons of light faster than you can blink.
Practicing with high ISO and fast shutter speeds is worth the payoff -- when you've captured that insane mid-air catch with perfect clarity, you'll know you've nailed the essence of sports photography.