Which plugs the UK, the US and Australia use
United Kingdom: type G
In the UK, the plug has three rectangular pins. A Romanian plug will not fit the socket without an adapter.
United States: types A and B
In the US, the plug has two flat pins (type A), sometimes with a third round earth pin (type B). Mains voltage is lower than in Europe, so check the charger says 100–240 V.
Australia: type I
In Australia, the plug has two angled flat pins, sometimes with a third for earth.
Voltage, not just shape
Besides the shape of the socket, voltage matters. In Romania, the UK, Ireland and Australia the mains is 230 V; in the US and Canada it is 120 V, and in Japan 100 V.
Most phone, laptop and earbud chargers are built for any voltage from 100 to 240 V: they say "Input: 100–240V". For those, a shape adapter is all you need.
Appliances that heat, such as hair dryers, curling irons and kettles, are often made for 230 V only. In a 120 V socket they work poorly, and the other way round they can be damaged. Check the label before you pack them.
Other common destinations
In continental Europe, including Italy, Spain, France and Germany, the ordinary Romanian plugs fit (types C and F), though in Italy and Switzerland some sockets have their own shape.
Malta and Cyprus use type G, like the UK. Canada uses types A and B, like the US. New Zealand uses type I, like Australia.
If you are going somewhere new, look up the socket type before you leave; hotel receptions often lend adapters, but not always.
One adapter for the whole family
A hotel room has few sockets, and everyone wants to charge their phone in the evening. A travel adapter with several USB-C and USB-A ports, plus a pass-through socket for your laptop, does the job of four chargers.
Look at the total USB power: it is shared between the ports in use. For charging phones and earbuds overnight it is usually enough; for a laptop, use the adapter's socket with the laptop's own charger.
Safety
Do not chain adapters into each other and do not charge on the bed or under a pillow: chargers need to cool. Unplug the adapter when you leave the room and check for it before check-out: it is one of the things most often left behind in hotels.
What to look for in a universal adapter
A good universal adapter has plugs for every country you often visit, usually the UK, the EU, the US and Australia, sliding out of one body so you do not lose parts. Its USB-C and USB-A ports charge your phone and earbuds directly, without their own chargers.
Check safety too: a fuse or surge protection, the CE mark and a body that does not get very hot. A cheap adapter with no markings is not the place to save money.
What to pack
For a short trip, an adapter with several USB ports and a cable for each device are enough. For a longer trip with a laptop, add the laptop's charger, which you use through the adapter's socket.
Note your destination's socket type before you leave and keep the adapter in your hand luggage: if your suitcase is delayed, your phone still needs charging.
Your earbuds for the flight go in hand luggage too: the TESNA Free7 weigh 3.9 g a bud and have active noise cancelling, so they cut engine noise without high volume.
If you forgot the adapter
Ask at the hotel reception first: many hotels lend adapters. Some rooms also have USB sockets by the bed, fine for a phone and earbuds.
Otherwise, adapters are sold at airports and electronics shops, but usually at a higher price and with a single port. A multi-port USB adapter packed at home saves you both the cost and the search.
For the flight, see also active noise cancelling earbuds.