Other than a typo on 121.5, Avspook has it mostly.
NSEU: "These are also not designed to operate underwater. These send signals on VHF and UHF. The UHF, I imagine, will be picked up by satellite. The UHF signal contains the aircraft ID, but not a GPS position."
The 406 MHz signal is data burst, and Lat/Long is optionally and usually wired from the Nav system. The 406 transmitted to a satellite constellation provides much more accuracy and reliablity in finding the aircraft.
Satellite monitoring of 121.5 was discontinued in February.
All you need to know about ELT is at
Artex -::- C-126 ELT (121.5/243.0/406 MHz)
The C-406N is probably their most popular today.
GB