Hardtails
No rear shocks. Generally cheaper, lighter, simpler and easier to maintain. Good for local riding.
Dual Suspension
Squishy & Grippy. Confidence-inspiring bikes engineered to ride tough terrain with the best mechanical advantages.
Urban MTBs
Rigid. No front or rear suspension. Maximum simplicity. Shreddy.  Ride it to work. Pedal it to the trail. Shred some laps.  Cruise home. Sleep. Repeat.
Bikepacking Rigs
"Choose your own adventure" bikes. Bolt on some racks, load up the gear, and hit the wild.
Freestyle MTB
Bikes for jumping