Here's a HAM radio:
A section of spectrum is called a "band". There are many different bands allocated for different uses. Some are for simple consumer electronics, some are for the military, some for GPS, some for airplanes, boats etc.
Along with all of the others there's one set of bands called the "HAM" bands set aside for amateurs. Unlike CB (you know, what truckers use), to use these bands you need a license, but they are pretty easy to get, and the class is the only cost. Also unlike CB, HAM radios can be run at high power and be used to talk to people on the other side of the world, if you know what you are doing (I'm not this knowledgable yet).
It's very difficult to talk a long distance using just a little handheld radio. So, HAM enthusiasts (called HAMs) will run "repeaters". You send the signal to the repeater on one channel, and listen on another. The repeater repeats anything it hears on the input channel on the output channel, but at much higher power (technically this is a "duplex" repeater). This lets you talk to people far away without having to send out that much power yourself or pull some other leet hack.
My radio and a hackI have a VX6R (the radio pictured earlier) that I've had since I got my technician's radio license something like 7 years ago or so. It's a small multipurpose radio that can do a hell of a lot of things across a wide set of different HAM bands.
Well, it turns out that Search And Rescue teams mostly don't use HAM bands, they use commercial bands instead. If you don't have one, this is a pretty cool one for that purpose:
My radio can listen on these bands, but it can't broadcast. But I want to be able to use my radio for SAR stuff. So what to do?
A quick search on the web, and it turns out all that's required is removing a single solder joint. A solder wick and iron later and my radio could broadcast on commercial frequencies. I just finished programming my radio this evening to to handle all of the channels I need for SAR.
Note - Do NOT do this hack if you don't have a specific legal reason to broadcast off band. Otherwise you're just setting yourself up to screw up and accidentally broadcast on a channel you aren't allowed to. IANAL but doing that can get you in trouble with the FCC... and yes, they really do bother to come after people.
Other usesMy particular use-case involves SAR, but radios can be pretty useful in lots of other situations. Mountaineering expeditions often carry a radio so they can call down to base-camp and give updates on progress or call for help. A lot of guides will carry a radio so they can call for help if needed, and sometimes will be the help as well. After Hurricane Katrina the communications network used both prior to and after FEMA's arrival was a HAM network set up by volunteers.
Just carrying a radio doesn't help, you have to know how to use it. If you consistently go to a particular area you can program in the useful channels in that area or just write them down. Some people also carry a repeater book listing all the local repeaters (note that you can find pdfs free). HAMs familiar with their radios may also be able to scan across a wide range of frequencies looking for anyone talking and then call for help on those channels.
A HAM radio, in my opinion, is anything but a necessity for going into the back-country. When I go out I rely on me, and generally don't assume I'll manage to get help. But that's my approach, and to each their own. And damn radios are pretty fun to play with and more useful the more organized of activities you take part in.