North Korea -- officially called Democractic People's Republic of Korea -- occupies the northern part of the Korean Peninsula.
Korea was a single country, ruled by the Japanese, until the end of World War II, when the north was occupied by the Soviet Union, and the south was occupied by the United States. These two were unable to reach an agreement and in 1950 the north attacked the south, starting the Korean War.
Today, the peninsula is cut in half by the Demilitarized Zone, a 248 km long by 4 km wide strip. North Korea is ruled by a military dictatorship which Amnesty International accuses of having one of the worst human rights records of any country on earth.
North Korea on Wikipedia