@Ties summed it up pretty well. A couple of things to clear up though. CB even though it’s 40 channels, only 6,19, and 21/22 (depending on the area) can become congested. And when I say can become, I mean they used to be back in the 1990’s, but CB here in the northwest is almost non existent... my Galaxy 959DX has been on for the past 18 months set to channel 19 (the unofficial “trucker channel”) with about one contact per quarter. So in other words very seldom used.
Second thing to clarify. It is illegal to use any radio not intended for GMRS on the GMRS frequencies. This includes using a Yaesu FT-4XR unlocked to transmit on the GMRS frequencies. A typical amateur radio walki-talkie puts out 1 watt on its lowest power setting, GMRS is 1/4 watt at the lower channels.
So without getting into the nitty-gritty, all CB radios put out 4 watts of power, are actually only made by two manufacturers who outsource the boards to other companies, so a $35 radio is the same performance as a $350 radio. No data signals allowed, repeaters or linear amplifiers.
FRS radios are hand held only but can communicate with the GMRS radios without a payed license. No repeaters, amplifiers, or data signals allowed.
GMRS. Limited to 50 watts of power. Ground Repeater capable, limited data transmission (GPS location only).
Amateur Radio limited to 1500 watts with a power amplifier, you can transmit data, and you can use repeaters (to include satellite and Low Orbit Reflectors).
The best setup is to have all three. I have all three but mainly use 2m amateur band for trail coms. One othe thing to note about repeaters, there are only 20 public repeater systems in the Northwest for GMRS use. While there are over 700 2m public repeaters in Washington state alone.
For more information on radios, check out the communication forums, and the Communication Boot Camp Threads. If I have confused you even more feel free to DM me... or find me on 3.885.50 MHz AM.