Michelin has a great site for bike tires which advises on which tire to get based on terrain:
example here:
http://two-wheels.michelin.com/2w/front ... 23&lang=EN there is a spider diagram which shows: rocks, hard, soft, sand, mud, durability. Each tire is rated so you can work out what you need.
It also has categories like Motocross, Enduro, Trials, Trails, and Rally / Raid.
Very helpful.