The "B-roll" got its name from early
TV news programs, though it technically came from the
film world. Back in the day, news stories were shot on film and played back live on the
broadcast. The editors would assemble an "A" roll or reel which had the primary sound, narration, and interviews. They would also assemble a "B" roll/
reel that had all the visual elements and cutaways and
visual aids that were relevant to the story. As the
reel played live on the
broadcast, a
technical director would switch between the "A" and "B"
reels in order to tell the story on the news.
Today, the news footage is edited together, but the name stuck. Any footage that's used to visually illustrate a story is now known as a "B-roll." This includes, slides, on-screen quotations, and archive footage. It also comes in handy as a cutaway shot if you need to cover an edit, or to hide a mistake an onscreen character makes by switching to another scene.
Source:
http://www.texasb-roll.com/ (They sell B-roll footage of Texas)