My favorite shots are always candids, and there is nothing more unposed and organic than sports photography. In all honestly I can't say whether sports photography is harder than portrait. I learned to use a camera doing sports photography. As it's my first, I found it a task to learn, and transferring what I knew to portrait photography was very simple. Motocross tends to be an easier sport to photograph. The riders follow a set line on a track, it is easy to track them with a camera even when they are running high speeds. Other sports, like soccer and football are a bit harder since the location of the ball is a bit hard to predict.
Here is a huge tip for sideline watchers, with phones, point and shoots or DSLRs.
Hard focus on an spot
Follow the athlete with your camera
Take a burst of photos
Get a solid focus on that point follow the athlete with your device.take a few rapid shotsPick a spot the athlete is going to be over. As they reach that spot, (either by holding the shutter halfway down, or click and hold on that spot on your phone). Then watch on your screen or viewfinder, and as they pass it. On the iPhone if you just hold down the shutter it will take a burst, and some cameras come with a continuous shooting option, and others you'll just have to go ham on the shutter. It's a trick that has served me very well. @armandofehr