Korea had been occupied by the Japanese for nearly fifty years before WWII. After the war, the peninsula fell like Germany; half to the U.S., half to the Soviet Union. Both sides were armed and organized by their patrons, and set up as independent nations.
On 25 June 1950, at the prompting of the USSR, North Korea invaded the south, quickly overruning the defenders, and pushing them south. The initial American response, dubbed Task Force Smith, was plowed under by the North Koreans (I've seen their regimental colors hanging in the Military Museam in Beijing, a gift from Kim Il Sung to Mao)
They were pushed back to Pusan, where the line was held for some time. General McArthur's risky amphibious assault at Inchon, near Seoul, cut the North Korean invaders off from their supply lines, and they were decimated. They were in full retreat, and pushed back into North Korea, and then back to the Chinese border. At this time, there was intelligence mounting that showed the Chinese had mobilized to join the war. U.S. commanders refused to believe that the Chinese were preparing to enter the war until they had already crossed the Yalu river and began their assault.
Using suicidal human sea tactics (a favorite of Mao), they pushed the UN forces back to the 38th parallel, where a bloodily unobserved cease fire commenced. In 1953, a armistice was signed, and the DMZ was set.
Casualties were staggering. The US had some 50k dead and 100k wounded. The ROKs and North Koreans had several times that (don't remember off the top of my head). The PRC suffered almost 1 million casualties, and millions of civilians on both sides perished.
That's the down and dirty, but there is a lot that I left out. Anything in particular you wanted to know? About 20 percent of