Here are some sample spectrum allocations for the US:
Application Frequency band AM radio 530 to 1700 KHz FM radio 88-108 MHz TV channels 2-4 54-72 MHz 802.11B and G 2.4 to 2.5 GHz 802.11A 5.05 to 6.65 GHz
This chart shows all of the FCC allocations for the US.
The FCC licenses radio stations, TV stations, cellular operators, etc. to use frequencies in a geographic area. For example, KABC has the right to broadcast at 790 KHz in the Los Angeles area. A different licensee could have that frequency in, say, New York.
We allocate spectrum and limit power output in order to reduce interference. If, say, a radio station broadcasts on the wrong frequency or with too much power, it will interfere with others. This may even be intentional. For example, Wolfman Jack reached most of the southwestern US by broadcasting 250,000 watt signals from Mexico.
Certain frequency bands have been reserved internationally for industrial, scientific and medical (ISM) use. License free operation is permitted in these bands as long as radio power is kept below a limit. These frequencies are now used by many devices including microwave ovens, wireless telephones, and WiFi LANs.
Many people feel that more spectrum should be license-free.
Some argue that advances in electronics will eventually make spectrum allocation unnecessary. They contend that, for example:
In recent years, regulators have begun auctioning licenses off rather than granting them for free. Many large cellular companies have gone deep into debt at these auctions.
Some economists argue that spectrum licenses should be treated like any other assets, and their owners should be allowed to sell them on the open market.