The Avro Lancaster is a British four-engined Second World War heavy bomber made initially by Avro for the Royal Air Force (RAF). It first saw active service in 1942, and together with the Handley Page Halifax it was one of the main heavy bombers of the RAF, the RCAF, and squadrons from other Commonwealth and European countries serving within RAF Bomber Command.
The "Lanc", as it was affectionately known, became the most famous and most successful of the Second World War night bombers, "delivering 608,612 tons of bombs in 156,000 sorties." Although the Lancaster was primarily a night bomber, it excelled in many other roles including daylight precision bombing, and gained worldwide renown as the "Dam Buster" used in the 1943 Operation Chastise raids on Germany's Ruhr Valley dams.
Source: Wikipedia, "Avro Lancaster", available under the CC-BY-SA License.