No Longer The Domain Of Hobbyists
When Bitcoin was flying under the radar, it was possible to mine bitcoins with a PC or powerful graphics card. But time and the increasing popularity of bitcoin have brought more and more powerful, mining-specific devices (called ASICs) onto the network, increasing the difficulty and energy required to mine worthwhile amounts of bitcoin.
The number of bitcoins remaining to be mined also diminishes sharply as time progresses. (These "block reward halving" events happen about every four years. The next one is due in 2016). All this means mining as an individual isn't as cost-effective as it once was. Many end up paying more for hardware and electricity than they ever make back in bitcoin.
Most mining these days is done either by large groups of miners, called 'pools', or companies that are set up specifically to mine. If you do want to get into mining, This site has some good information on it- but be advised, mining is definitely not the hobbyist pursuit it once was.
Still Good To Understand
While mining Bitcoin isn't a profitable venture for the average person now, it's still good for people to know why Bitcoins are being mined, and what's being done when it happens- because it's is a very important part of what allows Bitcoin to function as a viable payment method, even though it is decentralized. The section on the blockchain has a page that describes the mining process, and explains how it helps keep Bitcoin's transaction ledger consistent and synchronized.