Korea was one nation with a single culture before the end of World War II. At the end of the war Korea which had been occupied by Japan was arbitrarily divided between the USSR and the USA. Remember the two were allies at the time.
The division was originally intended to be temporary but later the Cold War started and the Iron Curtain separated the Communist Eastern Bloc from the Western Bloc. Both sides wanted to reunify Korea under their own rule and the Korean War resulted. There was much loss of life but finally the division remained the same. Korea has remained divided ever since.   
Similar to the PRC and Taiwan, both remained equally brutal and repressive until the late '80s, when South Korea democratized following its fifth regime change. Meanwhile, after North Korea's brutal dictator Kim Il-sung died, his son, Kim Jong-il, made it even more repressive for his 20 year rule, and then his son, Kim Jong-un, made it among the most repressive regimes in the world, only rivaled by Eritrea, Saudi Arabia, and the DAESH. Also, in addition to South Korea, they also now want Japan and Guam.
How things are
Korea is divided into 2 parts, North and South.
- North Korea is a "Communist" paradise where the people are starving and are miserable slaves of a crazy tyrant called Kim Jong Un.
- South Korea is a capitalist paradise.