Dpreview.com has a great guide to explaining the HD Video feature which has become available on the latest professional and prosumer dslr cameras. This website list the specs of the latest cameras that have HD video capability as well as comparison tables that show how they match up against professional grade camcorders. A wide array of topics are discussed from clip limits, storage, and sensor size to shooting styles, audio, and video resolutions.
The full guide can be found at this link:
The big difference between camcorders and digital SLRs is sensor size: this allows for better low light capture and the ability to shoot with really narrow depth of field for creative effect. By comparison most compact stills cameras have sensors somewhere between the 1/3″ and 2/3″ CCDs in the above diagram. – www.dpreview.com