Yes tax is paid at 18.7% (IIRC) and the advertised salary is net.
Your contract agency will provide you with documentation that you can use to claim tax relief in a country where you are resident for tax purposes. Worth pointing out that some countries have taxation agreements where you only pay tax in Korea with some other states wanting their fair share too. It's worth taking time to study the tax laws carefully that will affect you, since anything you can do to legally limit your tax exposure will literally pay dividends!
I see you are in Europe so off the top of my head, Ireland, Italy, Austria and Denmark are countries where you have no tax obligation to pay at home. There may be other countries too.