The BLU-114 (and the updated version, the BLU-114/B) falls under the category of non-lethal bombs, and are also known as "Graphite bombs". Its old designation is CBU-94, and was known as the "Blackout bomb". It is developed for - and used by - the US Air Force.
The bombs are basically a lot of steel wool packed into plastic cover and a small parachute, which is precision-dropped above electrical transfer-stations and high-power lines. When above the target, a small explosive charge makes the canister come apart, and disperses the strands of carbon-fibre wires. The wires are so fine that they "float" downwards in a web-shaped pattern, to maximise the chance of hitting the target.
The purpose of the bomb is to shut down electrical installations by short-circuiting them: When used in Serbia in May 1999, a set of BLU-114 bombs took out 70% of the country's power grid.
Sources:
http://cryptome.org/blu114-yu.htm (also has pictures and a graph)
http://www.fas.org/man/dod-101/sys/dumb/blu-114.htm
Guinness book of World records 2000 ("Most destructive non-lethal weapon")