There is nothing like the Korean Demilitarized Zone (DMZ) on earth. The DMZ cuts the Korean peninsula in half roughly along the 38th parallel. Approximately 160 miles long and two and a half miles wide, the DMZ was established to allow a ceasefire between the two groups vying over the territory. Today, the Republic of Korea remains south of the line, and the Democratic People's Republic of Korea remains north of the line. Rarely do the two sides meet, but one area where this does sometimes occur is at the Joint Security Area (JSA) approximately 30 miles from the heart of Seoul.
The USO runs tours to the base on the southern border of the JSA where roughly 100,000 tourists are escorted through per year to catch a glimpse of North Korea. Prior to the tour, all persons are required to go through a safety orientation and sign a waiver acknowledging the dangers of entering into a potential war zone.
The tours also guide people to various other spots in and near the JSA where major events occurred between the two countries. If you want to learn more about the DMZ and its history after your visit, the War Memorial of Korea is a good place to start, or you could pick up a book like Barbara Demick's excellent Nothing to Envy to read on the bus ride to and from Seoul and the JSA.