There are certain radio frequencies in the frequency spectrum that you can use without a license from the FCC (or similar), as long as you keep within certain predefined limits. These bands are generally referred to as ISM bands. The three letters stand for Industrial, Scientific and Medical.

The most common limitations are that you cannot take more than a certain width of the spectrum, exceed certain output levels, minimize your interference and accept interference from others.

A very typical example is the 2.4 GHz band where, among other things, Wireless LAN standards like IEEE 802.11, 802.11b and 802.11g are playing around. However they are not alone and have to accept interference from x10 video cameras, cordless telephones, garage remote controls, etc.