Ham Radio - or Amateur Radio if you prefer is a radio service in which you have to be licensed to operate your station.
Amateurs are given a portion of some of the radio bands from just above the frequencies used for Broadcast AM radio clear up into the GHz range.
Amateurs are given this privilege because they are supposed to perform a service, to create a pool of trained radio operators at no cost to the government that the government can call upon in time of need.
To understand what this entails, you need to understand what a radio wave is and how it is measured and how it is described by either its frequency in Kilocycles or Megacycles or the French term which is in Meters.
The most popular of these bands is the two meters band, at about 144 - 148 Mhz - just above the weather service frequencies and just below MURS and the State Police high band VHF frequencies. This only takes a Technician Class License to enjoy these privileges.