Ham radio is whatever you want it to be. It is more formally called Amateur Radio. In the early days, it was 'build a radio and see if you can send and receive a signal.' Today, you can still do that! It is different that broadcast radio in that you don't typically send out a signal for many to receive- it's a form of two-way communication, like CB, only without the limitations that CB has.
There are many modes of communication, and many more being developed; Amplitude Modulation (AM), Frequency Modulation (FM), Single Side-Band (SSB), Morse Code or Continuous Wave (CW), Phase Shift Keying 31Hz (PSK-31), etc. Nevermind what each of these modes mean for now, but let's talk about how the signals propagate: some frequencies can travel around line of sight- you can talk to someone in town... or directly above you. I have talked to the Space Shuttle Discovery on STS-56, and many talked to the MIR and International Space Station. Others signals you can bounce off the ionosphere for increased distance- I talked to an Chatham Island (off the coast of New Zealand) using only 25W of power FROM MY CAR! Some even bounce signals off the moon; called moon-bounce, or EME (Earth-Moon-Earth). I haven't done this mode, so I can't speak on it to any degree of accuracy.
So why do you want to talk to these people on the radio instead of the wonderful internet? In the 60's it was a thrill to talk to someone from another country- my dad had a good friend about the same age in Puerto Rico that he would talk to in Morse Code. Now, he can do that on the internet. Ham Radio's use comes in two forms: 'keeping the old ways' and 'independent communicaton.'
Keeping the old ways, I just mean, doing things the way it used to be done; the same reason people restore old cars, go sailing instead of a motorboat, ride horses instead of cars- it's a hobby. There are contests to see how many people you can talk to in a short amount of time. These are fun to comete in (very hard to win). There are also certificates you can receive for working someone from every state, every county, and numbers of countries in 100 block increments.
Independent communiciation means I have no 'service provider.' If the world collapsed today, all I need is my radio, antenna, and some power source (car battery, wind generator, solar panel, etc) and I can talk to anywhere in the world. During the September 11 attacks, cell phones were almost useless due to the towers being jammed with signals, phonelines going through the WTC area were destroyed, etc. Ham Radio does not have that problem. This brings up another good point: Emergency Communciation. In November, a line of storms ripped through Tennessee and Kentucky. I want to say it was like 35 tornadoes. Communications were completely cut off. Teams from the Amateru Radio Emergency Service (ARES) were called in and assisted with locating/rescuing people, restoring communications, etc. ARES mostly gets called out for floods (thankfully tornadoes and terrorist attacks re uncommon), where during a flood, all communcations are knocked out. Try surfing the net when your service provider is underwater!
Skywarn is another big piece of ham radio. Skywarn is an organization tied to the National Weather Service, where amateur radio operators will notify the NWS via the radio when severe weather strikes.
One important point to make about amateur radio is that it is a volunteer service. It's called AMATEUR because you cannot be paid for your services. You also cannot use amateur radio to say, have your company vehicles communicate with each other- there are commercial radios for this purpose. Even though we are amateurs, we act as professionals on the radio- there is no cursing or vulgar language/content on the radio, legally. It it MUCH cleaner than CB.
If you are interested, you must be licensed. There is no age requirement on the license, just as long as you pass the tests. There are four tests, and the priviledges (frequencies) you are allowed to operate on depend on your licensing. The first and easiest to get is called a Technician Class. These frequencies are in the UHF/VHF range, and once you pass this test, most of the frequencies used on Skywarn are open to you. These frequencies are fairly local- other end of town type frequencies. The other classes are General and Extra, and in order to get those you must pass a test for General and Extra, plus pass a 5 words per minute Morse Code test. These will give you HF frequencies capable of talking anywhere in the world! For sample tests, go to www.qrz.com and select 'practice tests.' They are multiple choice and it's not too hard to memorize the answers. If you have any questions on it, I will have an answer for you!