Suspension technology is kind of like computer technology - performance keeps improving with more features while picing is more affordable. Most of the maintenance and design problems of early forks have been worked out, but the principles and trade-offs are much the same. Long travel is great for fast downhills, but can come at the expense of efficiency and weight if you are going cross country. The latest suspension forks make the bumps fade away, don't bottom out, and need little maintenance.
Early mountain bike fork designs used motorcycle type oil dampening to slow fork rebound and compression rates. However, maintenance issues like leaks and adjustment difficulties mean that light and reliable oil dampened forks tend to be the most expensive.
Elastomer is a great low maintenance material that works like it sounds. Softer elastomers are (obviously) softer, and dense elastomers absorb the big hits. Combining them together and you get a fork that can take the hard bumps while still being sort of plush. However, elastomers tend to get harder the colder it is, they can have iffy rebound, and tend to be slow to respond with an uneven spring rate, as well as being on the heavier side. The advantages of elastomers and oil-damping or coil springs can be combined in a hybrid fork. Rock Shox's Judy forks use oil damping to tune the compression and rebound.
Air spring shocks use air "spring" cartridges, and sometimes air dampeneing. Air flows between two chambers, regulated by a valve you can adjust. This design is very robust, and very tuneable for rider weight and riding conditions, allowing for nearly full travel. Air springs are light, and combining air/oil makes for a lighter (and pricey) shock that is great all around.
For more in-depth on suspension forks, check out MTB Review's chatty fork reviews.
MTB Review '04 Front Shocks