General Mobile Radio Service
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The General Mobile Radio Service (GMRS) is a land-mobile UHF radio service in the United States available for short-distance two-way communications to facilitate the activities of an adult individual and his or her immediate family members, including a spouse, children, parents, grandparents, aunts, uncles, nephews, nieces, and in-laws (47 CFR 95.179).
Normally, as GMRS system licensees, family members would communicate among themselves over the general area of their residence or during recreational group outings, such as camping or hiking.
Source : http://wireless.fcc.gov/services/personal/generalmobile/
GMRS radios are typically handheld portable devices much like Family Radio Service (FRS) radios, and share some frequencies. Mobile and base station-style radios are available as well, but these are normally commercial UHF radios often used in the public service and commercial land mobile bands. These are perfectly legal and their use is encouraged. They are more expensive than the walkie talkies typically found in discount electronics stores, but are higher quality.
There are 7 "interstitial" channels shared with Family Radio Service, and 8 channels exclusively for GMRS. GMRS use requires an FCC license in the US, and licensees are permitted to transmit at up to 50 watts on GMRS frequencies (1 to 4 is more common), as well as have detachable antennas. GMRS licensees are also able to use the first 7 FRS frequencies (the "interstitial" frequencies), but at the lower 5 watt maximum power output, for a total of 15 channels. Radios programmed for GMRS may also use repeater systems. FRS channels 8 through 14 are not available for GMRS use; use of these frequencies requires an FRS transceiver.
More information at The Personal Radio Steering Group (http://www.provide.net/~prsg/wi-gmrs.htm)
Recently, consumer radios have been introduced that have 22 channels, instead of the 14 channels associated with FRS. All channels above channel 14 are GMRS channels and require a license. It is the responsibility of the radio user to read and understand all applicable rules and regulations regarding GMRS.
The following website provides basic consumer information about these consumer-grade radios: http://geocities.com/johnlwilkers@sbcglobal.net/gmrs
GMRS in Canada
In Canada, hand-held GMRS radios up to 2 watts have been approved for use since September 2004. Typically these are dual FRS and GMRS units, with fixed antennas, and operating at 2 watts on GMRS and 0.5 watts on the FRS-only channels. A licence is not required in Canada for operation at 2 watts on the GMRS channels. Mobile units, base stations and repeaters are not currently permitted on the GMRS channels in Canada.