GMRS stands for General Mobile Radio Service. It is a specific range of radio frequency channels in the UHF (Ultra High Frequency) spectrum, specifically 462Mhz to 467Mhz. An FCC (Federal Communications Commission) license is required to use GMRS, but unlike Amateur (Ham) Radio, a GMRS license from the FCC does not require a technical knowledge test. Many consider GMRS and Amateur Radio a good combination. Not GMRS licensed yet? Get started here.
Over a period of many years, the license fee has declined from nearly $100 to the current $35 (good for ten years), fee which covers your entire family (Part 95.1705(c)(2) Any individual who holds an individual license may allow his or her immediate family members to operate his or her GMRS station or stations. Immediate family members are the licensee's spouse, children, grandchildren, stepchildren, parents, grandparents, stepparents, brothers, sisters, aunts, uncles, nieces, nephews, and in-laws).
With this license, you will be issued a callsign (ex WABC123) from the FCC that you must transmit for Identification, at time intervals specified in the rules, when using the system (Following a single transmission or a series of transmissions; and, After 15 minutes and at least once every 15 minutes thereafter during a series of transmissions lasting more than 15 minutes.)
FRS is the Family Radio Service, and it is another division of the radio spectrum offered by the FCC. FRS and GMRS actually share some of the same channels (frequencies), but FRS does not require a license. The "Bubble Pack" units you would find at Wal-Mart or other retailer are typically FRS radios. GMRS radios can communicate with FRS radios on the common channels.
This is a reasonable question. The two services are interoperable, to a degree, but your GMRS license, and your GMRS radio, is far more capable than any FRS radio. FRS radios are limited to 1/2 watt or 2 watts, depending on the channel. FRS radios are not capable of communicating with repeaters. GMRS radios, on the other hand, can range from 5 watts (handheld/"walkie talkie") to 50 watt mobile and repeater stations. Additionally, most antennae can provide some "gain" or increase in performance resulting in the perceived multiplication of output power. Most FRS commercial advertising has somewhat exaggerated range claims. Occasionally up to 38 miles! In actual usage you'll get a mile (if conditions permit). GMRS radios can actually communicate many miles, and in ideal conditions, achieve 60 miles distance (from the repeater) or more with the aid of an elevated repeater, again depending upon your location and other conditions.
Displayed below is a Frequency Chart offered by BuyTwoWayRadios.com Get you own copy for Base or Mobile handy reference.
Relating to the channels/frequencies listed above there are a few restrictions that apply for those living in the northern U.S. near the Canadian border. Often referred to as "Line A" the details are here.