As part of emergency preparedness, it's valuable to have one or more emergency communication devices at your disposal. These allow you to call for help or signal your location when other mobile devices or communication methods have failed. They are often known as satellite trackers since they communicate via satellite.
Personal Locator Beacons or PLBs communicate with satellites to give emergency-only communication anywhere in the world. (Some only work in North America or Europe.) They can typically be tracked from an app or web site, which is nice for letting friends and family keep track of where you are, and have an "SOS" function that will alert a central monitoring center who will contact local authorities. More expensive models have the ability to send and receive text messages.
These require monthly subscriptions which can get very expensive, and due to the cost are primarily useful only for people spending extensive time in extremely remote locations. PLBs offer One-Way or Two-Way messaging.
One-Way
Unidirectional PLBs are cheaper, simpler to operate, and have longer battery life. However, communication is only one-way (out), and they may not be taken as seriously in an emergency. Common models: [Gen4, SPOT Trace]
These are generally simplified "Come help me!" beacons. Activating them will send a signal out to the company who operates them, who will contact local authorities. More advanced models have the ability to send pre-programmed messages to pre-programmed phone numbers or e-mail addresses, useful for sending out "Camped here for the night" GPS-tagged messages to friends or family.
A major disadvantage of these is the lack of ability for authorities to verify if the emergency beacon being sent out is actually an emergency. The S&R industry estimates that that between 80-99% of activations of these devices are false alarms, either because the owners didn't understand how to use them or because they had a different idea of what constituted an emergency.
While in a perfect world all emergency requests are taken as seriously as any other, the reality is that emergency services get "alert fatigue" when they get yet another call from the PLB company that will (probably) end up in a bunch of wasted effort.
Two-Way
Bi-directional PLBs are more expensive, but they allows for two-way communication, including more detailed or fully custom messages. They can also receive data such as weather reports, and may be taken more seriously by emergency services. Configuration and operation can be complicated, and they need to be recharged regularly due to higher power requirements. A PLB with a dead battery isn't much good to anyone. Common models: [Garmin InReach]
The major advantage of these is that they allow emergency centers to send messages back and forth to verify that it's an actual emergency. They also allow the sender to be more detailed about what their emergency is, which can greatly aid in rescue efforts. This makes it more likely that you'll get the appropriate help you need.
Additionally these devices allow for 160-charecter messaging (typically when paired via Bluetooth to a smart phone) to and from any phone number capable of texting. While it's minimal, some communication is better than nothing when you're in extremely remote areas.