EPIRBs (for vessels), ELTs (for aircraft) and PLBs (for individual people) are all
distress radio beacons, and all use the
International Cospas-Sarsat satellites.
As others have noted, one difference between an EPIRB and a PLB is that the EPIRB has longer battery life. A trade off is that they are much bigger than a PLB.
None of these devices can successfully transmit unless the antenna is out of the water. EPIRBs are designed to float, in a position with the antenna up out of the water. Many PLBs do not float, and even if clipped to your PFD, you would somehow need to make sure the antenna stays up and out of the water. There are some PLBs designed with a flotation cover, but they are larger.
EPIRBs (likewise ELTs) can be activated automatically. This is both an advantage and a disadvantage. If your boat sinks, it is obviously nice that it it will deploy and activate automatically. However, this feature leads to a high rate of accidental activation and false alarms. A PLB requires a deliberate conscious sequence of actions to activate, and there is a much lower rate of accidental unintended alerts.
Awhile back I had a conversation with someone who stands watches at the Alaska
11th Rescue Coordination Center. He said that when a PLB alert comes in, they are likely to immediately alert the appropriate agency to launch a rescue. When an EPIRB or ELT alert comes in, depending on where it is, they will sometimes take a bit of time to try to verify that it is a real emergency rather than a false alarm.