Full Suspension Bike
Full suspension mountain bikes are the choice of more than half of the people buying mountain bikes today. They make climbing easier in rough terrain because the rear wheel stays in contact with the ground for better traction, more than on a hardtail which gets bounced around. Descending on rocky terrain is better too, because instead of having to "pick a line" through obstacles, the suspension soaks up the bumps and keeps the wheels in contact with the ground. The rider will feel more confident and stable on a full-suspension bike than on a hardtail. Beginning riders feel confident negotiating obstacles that are more difficult on a hardtail, and experts love that stability too.
Suspension is set up according to rider weight, so that the rider gets the maximum benefit from both the front suspension fork and the rear shock. Both front and rear suspension can then be adjusted according to rider preference. Some riders like their suspension set up fairly "stiff", to give minimal shock absorption but greater efficiency (less "bob" and "bounce" on smoother trails). Some riders like it set up very soft, for a more comfortable ride.
Since there are so many different forks and rear shocks, the rider has plenty of choices as far as the adjustability of the system. Higher-end suspension (both front and rear) is more adjustable than entry level - great for racers who need to quickly adjust from riding a gravel road or a difficult technical section, and want the best performance from their bike in those areas.
Some riders like to "lock out" their suspension on climbs to improve efficiency. On smooth trails, the difference is very noticeable but in rough terrain, it's better to keep the suspension active for better traction. Many riders lock out their suspension when riding pavement.