EASY = These are usually considered rough fire roads that stock vehicles could easily conquer. Stock ride height, stock tires, open differentials and little or no previous four wheeling experience.
MODERATE = These are more challenging trails for a mildly modified vehicle. 2-3" lift, up 32" tires, limited slip differential in at least one axle, would have moderate difficulty traversing these trails. A so-equipped rig could expect minor body damage.
DIFFICULT = These are quite challenging trails, even for a well equipped rig with a novice driver. Rig improvements would comprise of 33-35" tires, 4-5" lift, at least one locking differential, winch, and a full roll cage. These trails usually have numerous difficult sections composed of large boulders and crevasses that are challenging for even an experienced driver to negotiate. Higher risk of body damage to occur.
EXTREME = This an extreme trail, generally reserved for very experienced drivers in very well equipped rigs. Rig improvements would include greater than 35" tires, Locking differentials in both axles, Greater than 7" lift, winch, rocker guards, body armor, full roll cage. Body damage and mechanical failures are likely. Extreme challenges that include massive boulders, vertical climbs, and pinch points.
EASY = These are usually considered rough fire roads that stock vehicles could easily conquer. Stock ride height, stock tires, open differentials and little or no previous four wheeling experience. 
MODERATE = These are more challenging trails for a mildly modified vehicle. 2-3" lift, up 32" tires, limited slip differential in at least one axle, would have moderate difficulty traversing these trails. A so-equipped rig could expect minor body damage. 
DIFFICULT = These are quite challenging trails, even for a well equipped rig with a novice driver. Rig improvements would comprise of 33-35" tires, 4-5" lift, at least one locking differential, winch, and a full roll cage. These trails usually have numerous difficult sections composed of large boulders and crevasses that are challenging for even an experienced driver to negotiate. Higher risk of body damage to occur.