Suspension isn't just about providing comfort to the rider on rough terrain, its also used to help the bike hug the ground to boost traction on climbs, in corners, and under braking. That means that there is a lot to think about when it comes to suspension.
Firstly suspension can be broken into two distinct types, coil or air. Coil suspension uses a metal spring and is known for smoothness and small bump plushness. Air uses the compression of air within the suspension as a spring and while they aren't quite as smooth, they give much more adjustability as the air pressure and volume can be easily adjusted. It's worth noting that coil and air shocks have another very different characteristic, coil has a very linear compression rate whereas air will ramp up, effectively making the shock harder to compress the more travel you use.
While premium suspension will certainly have more advanced internals, the biggest gain is in the adjustability that's offered. The best mountain bike forks and shocks will feature compression adjustments (how the fork reacts to an impact) and rebound (how fast the fork returns to full travel).
Suspension is probably one of the most confusing parts of a bike and they only perform as well as they are set up. That means if you don’t know what your doing you aren't necessarily going to see your moneys worth, on top of that suspension design and manufacturer's setup advice is better than ever so opting for a good fork with less adjustment doesn't always mean worse performance. If you're still trying to get a grasp of what those knobs do, check out our suspension 101 guide.