The two countries share their borders, their culture, but yet they are very different.
Due to the recent nuclear tests that were done in North Korea, UN has imposed some sanctions on the country. The United Nations expect countries to obey certain set of rules. Sanction is a kind of penalty imposed on a country if they disobey international law or rule. North Korea was warned by the UN not to do any nuclear tests. They ignored the warnings and went ahead with the tests.
After the sanctions were announced something strange happened. North Korea announced that all of their peace pacts with South Korea stand cancelled. And now it has also shut its hotline with Seoul and closed their shared border point.
South Korea’s newly elected President Park Geun-hye said the current security situation was serious, and her country will react strongly if their were any attacks from North Korea.
China which generally supports North Korea is asking both the countries to stay calm and peaceful.
North Korea had done a similar thing is the past, in 2010, so this is not entirely new.