B-Roll is video footage, often without a narration sound track, used by broadcasters to supplement the main elements of a news story.
In film and documentary making b-roll is seen as back-up footage. Wikipedia describes it as allowing the producer to "string together two interview clips that were not shot consecutively" by changing from the a-roll to the b-roll "while the audio from the A roll shot plays under. Then when the editor cuts back to the second A roll shot, it appears as if the concepts were always married together." 
The U.S. State Department describes b-roll as "video that enhances or tells the story. It was originally used to cover a jump cut in editing. It is now more commonly used to add dimension to a story. Example: If a person is talking about a new school, the B-roll will include video of the new school." 
In the context of discussion of video news releases, B-roll has a slightly different use. B-roll footage is usually provided to complement a video news release so that producers can use the provided visual material but add their own script and narration to make the story appear like it was all done by the news station.
On its website the VNR producer KEF Media Associates describes b-roll as "a 'picture story' is followed by a series of soundbites from spokespeople. All of the video is organized around 'slates' - text that offers editors and producers a suggested script and other information they need to turn the B-roll into a story for their program". 
In their 2005 annual report the world's largest producer of VNR's, Medialink Worldwide, describe b-roll as "supplementary and back-up material that is provided with the video and generally does not contain the narrated audio track, providing broadcasters with the ability to choose select portions of the video and provide their own voice-over."  (273kb Pdf)
Other SourceWatch resources
- Adam Hill, "RIP B-rolls?", PR Week, March 23 2006. (Sub req'd).