Water is served with all meals in Korea, but alcohol usually flows abundantly at dinner. Bottles get passed around, with everyone serving each other: except with familiar friends, it is customary that you don't pour for yourself. The (lousy) local beer is very popular, but the firewater of choice is soju, a sort of sweet vodka that you drink straight. It's only one small step up from rubbing alcohol and is drunk in shots. Considering that a 12-ounce bottle costs around $3.50 in a restaurant or less than a buck in stores, few people sip slowly. This is probably the reason that Jinro Soju outsells all other liquor brands in the world. Considering that it's a very local commodity (unlike Bacardi rum or Johnnie Walker Scotch), the per capita consumption rate is staggering--in more ways than one.
Korean drinking habits go back a long way: while one Shilla dynasty king was in the midst of a drinking game with his soldiers, a rival attacked the palace and quickly defeated the inebriated rulers.
Another popular beverage is makkoli, which is a milky fermented rice beverage that tastes much better than it looks. You will generally find it in more traditional restaurants and bars, with big clay crocks of it served in old-style Korean houses or in log cabins with fireplaces. You can also find a variety of unique folk liquors distilled from local fruits and flowers.
If you don't drink alcohol, you'll be considered a wuss, especially if you're at a business meeting. The idea is to get hammered with your friends and there is no stigma about 'holding your liquor. If everyone falls down or passes out while walking home, it's been a successful night!