Just five years ago, the seatpost category of mountain bike components was pretty bland: it consisted of short metal tubes that hold up bike seats. While those static seatposts are still a major product on the market, the real story in this gear category is the advent of the dropper post.
Dropper posts, also known as "seat droppers," allow the rider to quickly drop the saddle out of the way, usually by use of a handlebar-mounted lever. Dropping the saddle out of the way provides more room to move above the bike in rough terrain, more room to lean the bike in the corners, and it also allows the rider to get his center of gravity as low as possible.
Some dropper posts are completely mechanical, with cable actuation and hydraulic inners. Gravity Dropper produces excellent completely-mechanical dropper posts. Other posts have hydraulic inners but are still cable actuated. KS makes many excellent seatposts like this. And finally, some dropper posts are completely hydraulic, with hydraulic inners and hydraulic actuation. Rock Shox's Reverb line of dropper posts is completely hydraulic.