1. Ghosts: People who have a grudge when dying will often stay behind as ghosts. Some seek to kill the source (other person) of the grudge, and some will just look to harm/kill random people. There are friendly ghosts and evil ghosts. Buuuttt of course, evil ghosts make for better stories than friendly ghosts!
2. Usage of salt/alcohol: Salt and alcohol are often used as purifying items to drive away ghosts.
3. Buddhist references: Korea and Japan have strong ties with Buddhism, so you will see a lot of Buddhist references, such as chants, shrines, praying beads, etc.
4. School system: Korean/Japanese school year begins in the spring and ends at winter. Summer break is short, while the winter break is longer.
5. 2ch: a Japanese textboard. More info: http://en.wikipedia.org/wiki/2channel
6. Sleep paralysis: It's where during your sleep, your body seems to be frozen in place while you are fully awake. Most commonly, you witness ghosts or demons in such a state. It's not very common with Westerners, but many Koreans and Japanese suffer from sleep paralysis.
For more info: http://en.wikipedia.org/wiki/Sleep_paralysis
7. Naver: a popular Korean website similar to Yahoo! The search website also hosts webtoons, news, personal blogs, etc.
More info: http://en.wikipedia.org/wiki/Naver