All spiders are poisonous in the sense that they all have venom. Evolution has designed the venom to work on flying insects, so the venom is mostly harmless to humans. Of the 4,000 species of spiders in the world, approximately 100 can inflict a painful bite on a human. Only a few of these are found in Korea, and every one of them was originally imported from another country.
There are no truly indigenous Korean spiders that are harmful to humans--all the dangerous ones came from somewhere else. Before 1900, the majority of contacts with other countries were with China and Japan. In the 20th century, Korea became connected by aeroplanes and ships to every country in the world and so has unwittingly imported poisonous spiders from many countries--especially the United States and Australia. Spiders are very good at hiding, and sometimes this has afforded them a free trip to Korea.
The poisonous ground dwellers (wolf, tarantula) mostly came from China. Many of the poisonous web builders (brown recluse black widow) came from the United States--especially during the Korean War in the 1950s. The most deadly import of them all was the Australian funnel web spider. A single bite from a funnel web spider is almost certain death unless the subject is very near a hospital that has the rather rare antidote and is diagnosed quickly and accurately.
China is the only country that has a border with Korea, although Japan is only a short boat ride away. Wolf spiders very probably came from China long ago, and tarantulas probably came from the tropics via China or Japan. Black widow and brown recluse spiders almost certainly came from the U.S., and the funnel web spider definitely came from Australia. Korea has been more open to foreign contact than either China or Japan, and has more poisonous spider than either of these neighbours.
Brown recluse spiders have a darker brown, violin-shaped marking on the top. Their six eyes are arranged in three pairs. Black widow spiders have a red (sometimes orange or yellow) mark on the underside of their abdomen. This marking is usually in the shape of an hourglass but may be a bar or a dot. Tarantulas are very large and hairy. Not all tarantulas are dangerous, however.
The wolf spider is various shades of grey with a pattern that is reminiscent of the Union Jack (the English flag). It is a ground dweller that carries its children on its back. The Australian funnel spider can be identified by its unusual funnel shaped web. There are several species of this spider, but they are all darkly coloured without patterns. They have enormous fangs that are powerful enough to penetrate fingernails, clothing and some shoes.
Many spider bites that would not themselves be dangerous become infected because they were scratched with dirty fingernails. Insect bites should never be scratched. They should be gently cleaned, disinfected and covered. Seek medical attention immediately if it is a spider bite. If you can, bring the spider or a description of the spider. The biggest misconception about spiders is that they are insects that serve no useful purpose. They are not insects (they have eight legs, not six) but they are the major insect predators on the planet. If all the spiders disappeared today, we would be overrun by insects tomorrow.