HAM = Amateur Radio Operator as defined by FCC part 97 rules.
There are currently three possible licensing levels, Technician, General and Extra. As of last night, I passed the General test. Starting a couple of years ago, you do not have to learn morse code to get licensed.
With the general license you have almost full access to all the frequencies which are split into 16 bands ranging from 160 meters starting at 1800 kilohertz up to 23 centimeters starting at 1240 Megahertz.
The Technician license provides priveliges on the UHF (Ultra High Frequency) and VHF (Very High Frequency) bands of 50 MHz and up. On these bands there is a lot of local / regional traffic.
The lower frequencies, or HF (High Frequency) bands cover a much longer distance and depending on ionospheric conditions allow "skip" or bouncing of the radio waves potentially around the entire earth. It is possible to transmit and hear an echo comming around the world and back to you.
Another difference is antennas. Antenna size is related to the wavelength of the radio wave and typically are at least 1/4 wavelength and typically more. Wavelength in meters can be calculated by the simple formula 300 / frequency in megahertz. The higher UHF and VHF frequencies have much smaller antenna for example the poplular 2 meter band vs the 160 meter HF band. The 2 meter antenna fits on my small balcony, 160 meters, forget it!! Even a qubical antenna with 1/4 wavelengh sides would be over 128 feet on each side.
Lots of information on the ARRL website (http://www.arrl.org), the national association for Amateur radio, originally the American Radio Relay Legue.
Why the licensing??
The difference between amatuer radio and other radio services (CB and Family Radio) is that, in most cases it is not channelized, that is you can dial to any frequency. On top of that, subject to some restrictions to avoid conficts, you can use many modes that include voice using Single Side Band, Amplitude Modulation, Frequencey Modulation or Phase Modulation.. Data wise it might be RTTY, PSK, FSK, various packet protocals or even morse code which is considered a digital mode. There are also people bouncing signals off the moon, using amateur satellites and talking to the shuttle and space staion astronauts, many of whom are also hams.
Starting a few months ago, Nelson Winters, one of the organizers of FlaDotNet in West Palm Beach, FL was telling me about his other volunteer life as an amateur radio operator, organizer of a radio club in Boca and partiicpant in supporting Palm Beach government and other agencies during emergencies. There are two emergency organizations ARES and RACES with RACES focused on government and civil defense agencies and ARES everything else like the Red Cross.
This got me remembering when I was an emerging geek of about 10 years old (~ 1962)I use to go to an electroncs store in West Hartford, CT while my mom went shopping. Just around the corner from the store was the headquarters of the ARRL that I visited and thought about getting licensed but never did.
One interesting activity are "Nets". This is a time when people meet on the air and in time of emergency there is a formal format and messages can be passed all over the world potentially but most of the time is a chance for people to talk, announce meetings and activies and perhaps learn about some technical or other subject. Kind of an on-the-air user group meeting. I think the cell phone companies could make a killing with this. It does work however because there is a "Net Control Operator" who orchestrates "sign in" to the net and controls who is talking.
Another thing I have run accross is software defined radio. There is a company Flex Radio Systems (http://http://www.flex-radio.com) that sells a ham radio programmable with C#. Not sure i want to spend the few grand on it at this point but will take a look at the software.
I'm sure I'll get involved in one of the emergency services organization although my wife says if a hurricane comes I'll be with her on the way to Orlando!!
Ham radio groups, there are a number of clubs http://www.arrl.org/FandES/field/club/ around the country. Just like the user groups we are use to and a lot of nice people!!