I would like to elaborate a bit more than the links above offer.
Ham Radio (not HAM) is a hobby meant for people who like to tinker with radio communications equipment and see how they can use that equipment of various radio frequencies.
While CB is meant more for people who dont really care about the technical aspect (hence the reason for no license or test) and they just want or need to talk with there voice to a localized area for personal business (hence the reason CB only get a few channels).
Think of radio bands as different electric voltages, you have 12v that is in your car, 120v that runs your home TV, and 240v that probably runs your clothes drier. Yes they are all "electricity" but they are used for different things.
The ham radio bands are capable of doing different things, not just talking with your voice, but converting pictures into sounds and sending them bouncing across the world to someone in Japan, or transmitting computer data bits to an astronaut on the International Space Station. And only doing all this " just for the fun of it".
I would guess only an 1/8th of Ham Radio is picking up a mic and talking, you have to think about which band you want to use based on how you want the signal to propagate, once you know the band, which mode will work the best for the type of information you want to send. Then you have to know how to do all that without interfering with anyone else!
Personally, I use ham radio to assist with weather spotting activities, teach young kids and those young at heart the joys of this versatile hobby, and to keep in touch with my local ham buddies, and talk about the latest antenna or radio project.
Some people compete in on air contests, seeing how many unique stations they can contact in a give period of time. There is so must to do, and you would be suprised how easy it can be to apply ham radio to an already existing hobby or daily routine.
Steven KC9GMX