One of the coolest things about sports in action photography is that sense of blur that you can get from anything that isn't moving. It can be the crowd, it can be the street int he situation of this streetcar, unfortunately no named to desire in New Orleans. I took that photo screaming "Stella!" at the top of my voice, just kidding. It's one of the easiest things to do in photography, but one of the hardest things to get right and that's that sense of panning.
It's really important to have a relatively medium shutter speed, 1/6 of a second, 1/8 of a second, 1/10 of a second, or thereabouts, and track your subject perfectly.
As you're tracking your subject, and if you're steady, and keeping it straight, and working at the same speed, your subject will remain in focus, and the street will disappear. It's a lot easier with things like sports, where perhaps your subject is not moving as quick, but when you can get it right with something like a streetcar, or a car, or a plane, even better. So, blurring in sports and action, really easy to do, but it takes a little bit of practice to get there. In sports however, you might want that action frozen, and that's really going to come down to a very fast shutter speed. The faster the shutter speed, the faster you can freeze absolutely any moment in time.
You'll notice here on this photograph that we've made it so fast, that the ball is just visible, frozen in all it's glory, and keep in mind, in those pictures, they throw that ball pretty fast, 90 miles an hour, definitely, you don't want to get caught driving that fast. So, it's very easy to create life in a photograph, through panning, and freeze motion, and avoid blur, by having that really nice shutter speed.