A band is a group of frequencies. Sometimes, bands are grouped according to their wavelengths, in meters. The tuning locations of a station can be expressed as a frequency (kHz or MHz) or a wavelength (meters). Amateur radio operators generally refer to the frequencies they operate on by using the frequency's wavelength. For example, the 19-meter band refers to the range of frequencies with waves about 19 meters long.
Strike a piano key and you generate a tone. Pick up the tone with a microphone and your tone turns in to a "vibrating" or "cycling" electronic signal. The rate of vibration depends on the key struck. In electronics we refer to this rate of vibration as the number of "cycles per second." The formal term for this value is Hertz. As we move up in rate, such as in the Broadcast Band, we can use Kilohertz (KHz) to represent 1,000 Hz, or Megahertz (MHz) to represent 1,000,000 Hz. Continuing much further upward, we finally reach 1,000,000,000 Hz, which we can fortunately shorten to a Gigahertz (GHz). These frequencies are the home of both 802.11a (5 GHz) and 802.11b (2.4 GHz).