In the Star Trek universe, the Romulan Neutral Zone is a demilitarized buffer zone of indeterminate width established by the Treaty of Algeron following the Federation/Romulan War (also known as the 'tomed incident') of 2311. It's basically a plot device that drives home the Federation/Romulan cold war scenario. No more information on the war is currently known.
According to the treaty, both the Federation and the Romulan Empire are prohibited from sending any military forces into the zone (though scientific missions and/or probes are occasionally allowed) and that any ship caught in the zone is subject to immediate seizure by the opposite civilization. Owing to the Romulans' sophisticated cloaking technology (and the fact that the same treaty prohibits the Federation from developing cloaking technology of their own) the Federation has established bases on its side of the zone equipped with a tachyon detection system that acts as a subspace tripwire - any ship, cloaked or not, that crosses the border will be instantly detected. The Romulans have similar defenses erected on their side of the zone.
Many of the bases on both sides were destroyed by the Borg during their first incursion into Federation space; it is assumed that they have since been reconstructed. Despite the decline of Federation/Romulan tension brought about by their cooperation during the Dominion War, the Zone still stands in its former capacity.
There have been numerous violations of the Zone on both sides, though all of them have been stopped on the cusp of interstellar war by various turns of fate (including, on one notable occasion, the help of the Klingons). Regardless of the wording of the treaty, the standing policy seems to be that everything is reversible until shots are fired.