South Korea's standard electricity supply is 220V at 60Hz. Official Korea Tourism Organization guidance describes wall outlets as having two round holes. Travelers commonly encounter the two-round-pin European-style plug shape.
Before packing only an adapter, check the charger's input label. If it says something like 100–240V, the charger is designed for Korea's 220V supply and normally only needs a compatible plug shape. If an appliance is designed only for a lower voltage, a plug adapter alone does not convert voltage.
Phone, tablet and laptop power bricks are often multi-voltage, but high-power personal appliances such as some hair tools can be voltage-specific. Check each device rather than assuming.
If you arrive without an adapter, official tourism guidance says adapters can be found at places such as airports, duty-free shops, convenience stores and shopping centers. Hotel availability varies, so ask rather than relying on a loan adapter.