The currency of country of South Korea is the South Korean won, or won for short. The sign for the won is.
The South Korean won comes in coins of 10, 50, 100, 500. Bills or banknotes are available in denominations of 1000, 5000, 10,000, 50,000.
If you want to use your electrical items in South Korea then you will need to use a converter such as a Euro/German Shucko Plug or Type C Europlug.
Seoul is one of the safest cities on earth for both locals and tourists, especially for its size and large population. You should have no safety issues walking around any part of the city during the day or at night but always be aware of your surroundings as in any large city.