they are different in that a charger generally has battery holders for the batteries you will charge
a power unit plugs in to a device ...
then there is the device which has it's own batt built in ... and then the powersupply is also the charger
in your case
the potential output current is three times the recommended - this doesn't mean that the pu will force 300mA into the amp ... it just means that the amp COULD take 3x what it needs ...
the key number is the voltage
similar to your home mains
the ring supplies 13A @ 240v
it doesn't matter if it's a 3KW heater, or a 40W bulb
the safety is provided by a fuse appropriate to the device.