The hertz is the unit of frequency. One hertz is defined as one cycle per second.
It is named in honor of Heinrich Rudolf Hertz and is abbreviated Hz.
While there are many possible prefixes for the unit Hz, the only ones really relevant to radio are kHz, MHz, and GHz.
kHz = kilohertz = 1,000 Hz
MHz = megahertz = 1,000 kHz = 1,000,000 Hz
GHz = gigahertz = 1,000 MHz = 1,000,000 kHz = 1,000,000,000 Hz
See also WikiPedia:Hertz.