Amateur Radio - What is it?
Amateur radio (or "ham" radio) is a non-commercial hobby that encourages radio communications and experimentation. Ham operators are licensed by the U.S. Federal Communications Commission or the equivalent authorities in other countries. Historically, amateur radio has attracted technically-inclined people who are interested in building and operating radio receivers, transmitters, and antennas. Today, excellent equipment is available commercially at attractive prices, so hams rarely build their own basic receivers and transmitters. Many interesting technical frontiers remain to be explored, including for example, computer-based radio signalling methods, satellite communications, and new antenna designs.
Ham radio operations is the most interesting aspect for many. You can compete to work the most geographical areas ("DX" - long distance), you can help out in emergency and public-service communications, you can operate "mobile" from your car, boat, or airplane. There are many other special operating niches.
Amateur radio has a grand tradition from the 1910's onward, but the proliferation of computers, internet technology, cheap long-distance, and the cell phone have fundamentally changed the environment for the hobby. Many of the younger "would-be's" won't be, because of all the alternative outlets.
Still, there are over 600,000 licensed ham radio operators in the U.S., and active organizations like the ARRL, supporting everything from old-fashioned "rag-chewing", to data transmission, to moon-bounce and satellite based communication, and beyond.
[Try Amateur Radio in the Wikipedia.]