What exactly IS trail running?

It's like walking on a trail except faster.

Here is an example of a trail race.

 http://forums.about.com/n/pfx/forum.aspx?tsn=1&nav=messages&webtag=ab-running&tid=4751

There are a couple of things that make trail running/races different.  It also depends on the distance too. Alot of times on trail runs you need to be much more self-sufficient as far as carrying water and calories.  You also have to be much more alert to your surroundings because trips hazards are EVERYWHERE.  The hills are more extreme usually also.

I have done trail ultras but I still prefer road running.

Trail running is much better on the bones/muscles because of the softer trail surface, however, you must be watching the ground all the time to avoid tripping on trail roots, rocks, dips, etc. There's a definite acclimating process, however, when you first start trail running, getting your ankles used to the rough terrain, but in the long trail run, I think trails are more gentle on the body than smooth sinister asphalt. I love trails and road. When I get sick of the constant vigilance of trail running, I hit the roads for the nice soft bounce of the asphalt, and likewise, when I get bored with the roads, I hit the trails.  I think mixing in both kinds of running makes for an over all stronger runner.