There are many types of two-way radio services, and many subsets of those services. This article will detail multiple types of services and what they are used for. NOTE: This article is primarily designed with FCC regulations in mind.
The Family Radio Service, or FRS, is perhaps the most basic of the radio services. The FRS uses the same channels as the GMRS radio service, but doesn't allow for as much power. Additionally, the FRS only allows for handheld radios with non-removable antennas.
The General Mobile Radio Service, or GMRS, is a fairly recent radio service that is rapidly gaining popularity amongst offroaders, truckers, and many more groups. Its interoperability with the FRS allows unlicensed individuals to participate in communications. It also allows for handhelds and mobile radios, In addition to sharing 22 channels with the FRS, it also includes eight repeater channels for repeater communications, something not allowed for in the FRS.
The Citizens' Band radio service , usually abbreviated as CB, is an unlicensed radio service that is generally following a downward trend in terms of popularity. It uses the much lower HF band, requiring larger antennas for communication. While not neccessarily a downside, it is something to be considered. The lower HF frequencies allow for something called propagation, or "skip" in CB slang, which is essentially when radio signals bounce of the atmosphere, allowing for communication with individuals hundreds or even thousands of miles away.
The Amateur or "Ham" radio service is by far the most powerful radio service available to non-commercial or governmental organizations. It has many bands on many parts of the radio spectrum, and is gaining popularity due to the wide range of capabilities available with it.
The Multi-Use Radio Service, or MURS, is a fairly unused radio service which is similar to the FRS but operates on a different band.
The Marine VHF radio service is a service limited to boat stations and coast stations communicating with said boats. Stations are typically handheld or mounted on the boat.