What is the difference between the following styles of unicycling: trials, street, and flatland.
There is no difference, the names are just there to confuse you. 
Just kidding, try looking through this, there is a pretty decent description in it somewhere.
Trials is attempting to ride or hop over objects.
Street is attempting to do tricks while riding and hopping on and over objects
Flatland is doing tricks while on a flat surface.
Thank you. It helped alot.