plb's and epirb's

An EPIRB alerts the search and rescue services allowing them to quickly locate you in the event of an emergency. When activated it transmits a coded message on the 406 MHz distress frequency which is monitored by the COSPAS-SARSAT satellite system 
 

In a similar fashion a Personal Locator Beacon (PLB) is a personal electronic transmitter used to alert rescuers that there has been a life threatening man over board (MOB) situation with a need to be rescued.

When activated, the PLB sends out a signal on either a 406MHz frequency or Local Area System using 121.5MHz, VHF DSC and/or AIS.

For that extra piece of mind when out at sea on an inflatable boat, especially if you decide to go that bit further out than normal they could save your life.

The difference between the two:

PLB's are designed to be carried on the person so they are much smaller.

They're designed to be used anywhere in the world, on sea or on land. 

An EPIRB is registered to a vessel, whereas a PLB is registered to a person.