The Israeli West Bank barrier is a physical barrier being constructed by Israel consisting of a network of fences with vehicle-barrier trenches (95%) and concrete walls (5%).
The barrier is a very controversial project. Supporters argue that the fence is as a necessary tool protecting Israeli civilians from Palestinian political violence in Israel that increased significantly during the al-Aqsa Intifada in September 2000  and regard it as a major causal factor in reducing incidents of terrorism by 90% from 2002 to 2005. Opponents argue that it violates international humanitarian and human rights law, and see it is an attempt to annex land utilized by what they regard as illegal settlements, and to pre-empt final status negotiations between Israel and the Palestinians. Opposition to the barrier is focused on the route of the barrier and its impact on the Palestinians who live nearby, particularly on their ability to travel freely within the West Bank and to get access to work in Israel.
A similar barrier, the Israeli Gaza Strip barrier, was constructed parallel to the Gaza Strip portion of the 1949 armistice line in 1994. This barrier did not stray significantly from the armistice line, and it has not been as controversial as the West Bank barrier has been.
(See link: the purple line is the completed fence, the red line is the planned fence)