Operation Chastise was the official name for the attacks on German dams using a specially developed "bouncing bomb".
The attack was carried out by Royal Air Force No. 617 Squadron, and is widely known as "The Dambusters". A major allied strategy in WW2 was to attack the industry that fueled the Axis war effort, and several German dams were targeted. Not only did these dams provide hydroelectric power, but also provided water for industry and the canal systems. High altitude bombing was inaccurate and the German dams were also protected by torpedo netting which complicated low altitude attacks. So the "bouncing bomb" was developed which when dropped at low altitudes and the correct speed, would skip across the surface of the water and avoid torpedo nets.
In a fairly costly attack (8 of 19 planes shot down) the Brits were able to breach both the Möhne and Eder dams, and subsequent flooding killed 1294 (of which 749 were POW's) - Germany was able to get the dams back to full production much quicker than anyone figured. Probably the largest effect was that Germany had to deploy additional forces to defend the dams for the remainder of the war, waiting for another attack that never came.
Photograph (colorized) of the breached Möhne Dam taken by Flying Officer Jerry Fray of No. 542 Squadron from his Spitfire PR IX
Photograph (colorized) of the breached Eder Dam
Although the Dambusters mission wasn't a crippling blow by itself, combined with other attacks by the allies on all forms of German industry had a huge effect on the outcome of the war.
Here's a cool site with more about the "Dambusters"