What is the difference between a hard tail mountain bike and a soft tail? What is the benefit of either?

I am truly disappointed with the regular crowd not catching this and answering correctly. The others I'll just chalk up to lack of knowledge.

A HARDTAIL is a bike with no rear suspension. It is good for riding on road and off road. Not very good for coarse terrain.

A SOFTTAIL is a frame that typically has a short travel (25mm or less) rear shock and no pivot, allowing the frame to "give" just a bit to take the harshness off the ride. This is ONLY for comfort. For the record, I did see a softtail bike with a pivot but it had an aluminum frame which would crack in half without it. Examples of softtails are made in everything from cheap aluminum (Walmart Next) to high end titanium (Ibis Bow Tie).

A REAR SUSPENSION is intended primarily for offroad use although cheaper bikes use it as a selling tool. Rear suspension is not for comfort but to keep the wheel on the ground for better handling- something that is unnecessary when riding on the road and less technical terrain. They tend to be inefficient... every time you pedal a bike with rear suspension it flexes. When it flexes it eats your energy instead of producing forward motion. Since the strongest of riders only put out about 1/4 horsepower it would be foolish to use rear suspension unless you really need it.

a soft tail has rear suspension

a hard tail has no rear suspension and might or might not have front shocks.

it really depends on what kind of riding you do, and the price. It would be cheaper to get a hard tail, but if you are a good mountain biker and mountain bike a lot i would recomend a soft tail.

if you do get a soft tail just remember to tighten the suspension when you are road riding

Hard Tail:
No rear suspension.
Lower cost than a full Susp. bike.
Normally lighter in weight.
Less parts to wear out.
Rougher ride on bumpy trails.

Soft Tail: (full suspension)
Has rear suspension.
Higher cost for same component level.
More parts to wear out.
Much better ride on rough trails.

Key Note: A rear suspension is not intended as a comfort item, the reason for a rear suspension is to follow the trail surface!!!

Hard tail = no shocks
Soft tail = shocks

Hard tails have less weight usually, but Soft tails offer a better ride.