Blowout Preventer

What is a blowout preventer? In the Deepwater Horizon oil rig disaster, it is suspected that the blowout preventer failed. Please see Wikepedia's explanation given below:
BOPs come in a variety of styles, sizes and pressure ratings, and usually several individual units comprise a BOP stack. Blind rams are designed to close an open wellbore. Pipe rams seal around tubular components in the well (drill pipe, casing, tubing, or coiled tubing). Shear-seal BOPs are fitted with hardened steel shearing surfaces that can actually cut through drill pipe and tool strings, if all other barriers fail. Since BOPs are important for the safety of the crew, as well as the drilling rig and the wellbore itself, BOPs are regularly inspected, tested and refurbished. Tests vary from daily test of functions of critical wells to monthly or less frequent testing of wells with low likelihood of control problems.Any of these BOPs may be installed underwater, normally with two hydraulic actuators.
Deepwater Horizon blowout
After the Deepwater Horizon drilling rig explosion on April 20, 2010, the blowout preventer should have activated itself automatically to avoid an oil spill in the Gulf of Mexico. Underwater robots were sent to manually activate the mechanism's switch, to no avail. As of May 2010[update] it is unknown why it failed.BP representatives suggested that the preventer could have suffered a hydraulic leak. Gamma-ray imaging of the preventer conducted on May 12 and May 13, 2010 showed that the preventer's internal valves were partially closed and were restricting the flow of oil. Whether the valves closed automatically during the explosion or were shut manually by ROV is unknown.
Please see this link for a pdf version powerpoint presentation of BOP (large file-be patient!).