It would be a lot easier for someone to respond to a specific question as opposed to a broad request to explain several radio bands to you.
In a nutshell
FRS (Family Radio Service) is a group of low power (UHF) frequencies available for use without applying for any FCC license. Generally, these are the frequencies the walkie talkies you can buy in Wal Mart and sporting goods stores operate on. There are legal restrictions on the transmit power, antenna etc. I have neighbors with small farms - 15 acres and FRS works great communicating with everyone on the farm.
MURS (Multi Use Radio Service) is similar to FRS however operates in the VHF frequencies, allows transmit power up to 2 watts and detachable antennas. These radios are not quite as available - yet - as the FRS radios - but they are popular and are what a lot of Wal Marts use.
GMRS (General Mobile Radio Service) is a group of frequencies in the UHF band and generally requires a license to utilize. No test, they just want your money to use these frequencies. Radios can have a transmit power of up to 50 watts, antennas on towers are permitted, as are repeaters so the posibility of communicating over a much greater distance is available with GMRS radios. Depending on terrain and equipment, it's not unusual for 2 hand held units to be able to communicate to each other while they are 20 - 30 miles apart. Each one being 10 - 15 miles in the opposite direction from a repeater tower.