When it comes to suspension, mountain bikes (or really any bike for that matter) can be lumped into one of three categories: rigid, front suspension (aka hardtails), or full suspension.
Rigid Mountain Bikes
As the name implies, rigid mountain bikes don’t have any suspension at all. The frame is fully rigid and the fork is rigid too. With less moving parts, these bikes arguably require less maintenance than bikes with suspension and they’re also generally much lighter and less expensive. However, most people like to have at least front suspension, if not full suspension, for both comfort and handling.
Front Suspension Mountain Bikes (aka Hardtails)
The term hardtail refers to any mountain bike that doesn’t have rear suspension so, technically, rigid bikes are also hardtails. However, rigid bikes are generally referred to as either “rigid” or “fully rigid” and the term hardtail most commonly refers to bikes with a rigid frame and a suspension fork. Again, with less moving parts than full-suspension mountain bikes, hardtails probably require less maintenance and are also generally lighter and less expensive. The front suspension not only helps to smooth out the ride of rough terrain but it also helps improve traction and steering control by increasing the amount of time that the front wheel is actually on the ground.
Hardtails are popular with riders who prefer the relatively fast-paced cross-country style of riding, partly because the lack of rear suspension results in higher efficiency. This is because, on full-suspension mountain bikes, at least a little of the force exerted by the rider’s pedal stroke is absorbed by the rear suspension rather than being directed to the drive train.
Full-Suspension Mountain Bikes
In addition to having suspension forks, full-suspension mountain bikes also have articulated frames with an integrated rear shock. The variety of full-suspension frame designs is relatively mind-numbing and is constantly evolving. Full-suspension frames can have a single pivot point or multiple pivot points. As is the case with front suspension, the rear suspension also improves comfort and handling by absorbing the impacts of the trail. One of the disadvantages of full suspension is that at least a little of the force exerted by the rider’s pedal stroke can be absorbed by the rear suspension rather than being directed to the drive train. This reduced efficiency generally makes full-suspension mountain bikes a little more challenging when pedaling uphill. The amount of suspension (or “travel”), both front and rear, can also vary dramatically from one design to another. Cross-country style full-suspension mountain bikes will generally have the least amount of travel, with more travel on trail bikes and all-mountain bikes, and the most travel on downhill bikes that are not really designed for being ridden uphill (think lift-serviced downhill trails or downhill racing).
Hardtail versus Full-Suspension for the Same Price
If you are comparing a particular hardtail mountain bike with a similarly-priced full-suspension mountain bike, you need to keep in mind that it’s not exactly and apples to apples comparison. Full suspension frames are much more complicated and therefore cost more to manufacture than hardtail frames, plus there is the very significant additional cost of the rear shock. With significantly more money going to the combination of the frame and rear shock, there is less money left for the components. Therefore, when comparing a particular hardtail bike with a similarly-priced full-suspension bike, the hardtail will often have a better suspension fork, drivetrain, brakes, and even wheels.