Unlicensed or license-free spectrum as it is sometimes called simply means a spectrum band that has rules pre-defined for both the hardware and deployment methods of the radio in such a manner that interference is mitigated by the technical rules defined for the bands rather than it being restricted for use by only one entity through a spectrum licensing approach.
Any person or entity that does not infringe upon the rules for the equipment (which in practical terms is all pre-certified by the manufacturer) or its use can put up a license free network at any time for either private or public purposes including commercial high speed internet service.
Some of the most commonly used license free frequencies in the US are at 900 MHz, 2.4 GHz, 5.2/5.3/5.8 GHz, 24 GHz and above 60 GHz. Other spectrum just now gaining tracton is a small band at 3.65 GHz (with specific different rules set for its use). Much of the spectrum above 60 GHz (millimeter wave band) is unlicensed. There is also a band at 4.9 GHz which is allocated for public safety use. The rules vary from band to band.
Other countries have varying rules about license free spectrum. In the UK for example, the power allotment for 2.4 GHz is about one tenth that allowed for products sold in the US.
The rules for equipment in each band varies somewhat as does the power allotment and configuration of equipment. One example of license free service is the many Wi-Fi Hotspots that have sprung up in cafes, airports and commercial venues in recent years. The spectrum used for these Wi-Fi networks is mostly at 2.4 GHz and 5.2/5.3 GHz. In the US, the 5.2/5.3 band is used for both 802.11a hotspot access as well as outdoor use. The rules for these bands define a different power level and a more integrated antenna and radio than the 5.8 GHz band does. Any coffee shop in America, if they follow extremely simple rules can set up an unlicensed network.
Typically, the power allotment for each license free radio is much lower than that enjoyed by licensed spectrum holders. One real exception to this rule is the 3.65 GHz band, which allows up to 1 Watt per MHz of power output, higher than unlicensed spectrum. Technically the 3.65 GHz band is licensed, but the cost and rules defining the band and its use are so modest that it is effectively unlicensed spectrum. This band is already proving to deliver NLOS capability with very solid bandwidth capability. Power allotment largely relegates license-free providers to LOS coverage only. Although 900 MHz products feature limited NLOS capability at short ranges from 1-2 miles depending on tree cover and building shadow.