Flat and wide, often a road or walking path. Can be strenuous. No special skills required.
Moderate single track with gradient. Some structures, but low and can be ridden without jumping. Good hiking.
Steeper and tougher single track that may have higher structures or mandatory airs and larger drop offs. Strenuous hiking.
Large drops, and high, skinny structures, or steep slopes and exposed terrain. Very difficult to hike.