Tax ..nasty stuff try and avoid it where ever possible.
Normally as an expat in places like Singapore, Hong Kong, Malaysia if you take a post there and have to get a local work permit or residence visa you have to pay the local tax (Singapore about 12% average, HKG about 16% for example). Some places, like Indonesia the norm is that the company you work for pays the tax on your behalf. Other places the tax is totally your responsibility but in other cases companies will pay an end of contract bonus that roughly equates to the tax you have paid, so they kinda pay it for you.
You can try an negotiate various options, some companies are flexible and will pay you 'off shore' and just pay a local allowance on site, others less so.
Cant say as I know how Korea works, but I am sure there are korean expat web sites you can Google.
good luck