Trail running entails just what it says...running on trails.
It is a type of running that takes place on a wide variety of terrains, including dirt, grass and mountainous terrain, and often includes ascending and descending of varying degrees of difficulty.
In other words, it's faster hiking.
The trail running community in and around Phoenix is thriving. Runners in the area take full advantage of the trail system, and have many trail running events and races to participate in. While there are a few bigger companies that put on awesome events for all varities of runners (from beginner to professional), Phoenix also has it's fair share of local mom-and-pop race events that are all about having a good time on the trails.