They share the same 22 channels, and a $20 FRS radio will talk to a $300 GMRS mobile on channel 3. The differences are in what the radio is allowed to be, and what you're allowed to do with it.
| FRS | GMRS | |
|---|---|---|
| Licence | None | $35 / 10 years, no exam, covers family |
| Power, ch 1-7 | 2 W | 5 W |
| Power, ch 8-14 | 0.5 W | 0.5 W (FRS rules) |
| Power, ch 15-22 | 2 W | 50 W |
| Repeaters | No | Yes - the whole point |
| Antenna | Fixed to the radio | Removable, external, on the roof |
| Callsign | None | Required, every 15 min and at the end |
| Data / text | Limited | Limited (short text and location allowed) |
So which do I need?
If it's two kids on a campground, FRS. If you ever want to reach across town, up a mountain, or through a repeater, GMRS - and once one person in the family has the licence, everyone's covered.
Can they talk to each other?
Yes, on simplex (radio-to-radio) on any of the 22 channels, as long as the tones match or are off. The FRS radio just can't hit the repeater. A lot of families run exactly this: a GMRS mobile in the truck, FRS bubble-packs for the kids.