Emergency Location Beacons, or Transponders

COSPAS-SARSAT system

COSPAS-SARSAT is a satellite based system listening for the distress signals from the ground. A wide variety of the end-user devices (called an Emergency Location Beacon) exist, different for aviation, marine, and personal use. The emergency signal is then relayed to the appropriate search-and-rescue (SAR) team.
The system uses two types of satellites - Low Earth Orbit (LEO) and geostationary (GEO).
GEO satellites provide instantaneous detection for a 406 MHz signals. If the signal has a position data encoded (GPS or other navigation unit), the position is available immediately. Otherwise, an ID of a beacon is checked against the database, the beacon owner or other point of contact is called to check for a false alarm or for a details of a planned trip.
LEO satellites perform a Doppler-shift analysis of the signal to provide a location data. An average time to acquire an initial position fix is about an hour.

Emergency Location Beacons, or Transmitters (ELTs)

An ELT, when activated, transmits a distress call to a satellite. It also provides some sort of homing capability, to allow search and rescue team to pinpoint the emergency site.

121.5 MHz ELTs (older generation)

Currently being phased out due to low efficiency.
Originally developed to alert an aircraft flying overhead, hence use of the aviation emergency frequency (121.5 MHz).
Signal does not contain any information about the vessel requesting assistance. It just provides a beacon to home on.
Very high false alarm rate (up to 95%). On top of that, because no information is available other than position, each signal must be chased to its location, utilizing search-and-rescue (SAR) resources.
Initial position data is available in about an hour, with an average of 20 km accuracy.

406 MHz ELTs (current generation)

Transmits an unique identification number as a digital signal on 406 MHz. Additionally, transmits a low power 121.5 MHz "homing" signal for rescuers to determine a final location as they are closing in.
Signal may include encoded GPS position fix, if the device is so equipped.
Device registration is required (free of charge; if you are in US, you can do it online here). This allows false alarms to be filtered out with just a phone call ("Hello, you are the owner of the emergency transmitter? We are picking a signal, are you fine? - Uh oh, my three-old-year son just activated it! Sorry for a trouble. -- OK that's false alarm, have a nice day.").
The system is monitored by geostationary satellites, providing an instantaneous alert. If the device is GPS-capable, the search-and-rescue team gets the exact location immediately. If no fix is available, the Doppler-shift analysis is performed similar to the 121.5 MHz beacon, but with greater average accuracy of about 5 km.

Personal Location Beacons (PLB)

PLB is a small-size 406 MHz emergency location transmitter, sold by the outdoor sports, general aviation and marine outfits.
Size and weight similar to the mobile phone.
Must be registered with the authority before use (online PLB registration if you are in US).
The unit is priced at about $500-$600 (US). Use of the beacon is free, and there is no charge for an accidental false alarm. Deliberate misuse is a different story, subject to massive fines.