Accurate ommunication is important if for no other reason than it saves time. Unfortunately most people misuse three terms which are crucial to the relationship between Director and Editor. INSERT, CUTAWAY and B-Roll are common terms and most people don't know their real meanings. But you will after you read this.
B-roll is a relatively generic term so let's leave that for last and start with 2 terms often confused, misused and abused: Insert and Cutaway .
An INSERT is a specific term designating a shot as being relative to another specific shot. Imagine a shot of a man answering his cell phone. He looks at it and reacts, saying, "I'll have to leave." then exits. It begs the question, what did he see on his cell phone? As a filmmaker you may choose not to let the audience know and build suspense, or you may want to show the audience what the man saw. If you choose the latter, the close up of the phone's message would be an INSERT SHOT. When labeled this way (something like: Insert 1 take 1 to Shot 13A Scene 42) the editor will know where it goes and how the director expected it to be used. Almost all Insert shots require continuity with the scene they are matched to. You can't see the man pick up the phone with his right hand and on the insert see it in his left hand. Also by definition the Insert shot is designed to cut in and out, in other words the scene would show the man, the phone CU and return to the man. Of course, the editor will have options based on the coverage provided by the director and as I said, may choose to not even use it. But the insert shot of the phone will have a specific purpose related to another specific shot.
A CUTAWAY on the other hand is a shot not intimately related to another shot. Let's imagine two people talking on a busy street corner. There's lots of things happening around them which may or may not be important to the dialogue or meaning of the scene: people passing; cars driving by; stores with display windows, etc. It's all atmosphere but nevertheless related to the scene, just not intimately related as an insert shot would be. The filmmaker may choose to capture some of this ancillary activity just to give the editor some more footage to work with, let's say to simply enhance the atmosphere... or may have a very planned purpose: wanting to use these extra shots to help the editor create a sense of frantic distraction and interruption in the conversation. The latter purpose would change the tenor of the scene dramatically. But whatever the reason or the purpose, these additional shots would be labeled CUTAWAYS. They are shots happening outside, or behind, or around but not directly within the frame of the two people talking. The same designation of CUTAWAY would be more obvious if one of the two people referred to something, "over there," —perhaps a hotel across the street— because to use a shot of the hotel, you would be CUTTING AWAY from the speakers in order to show it. Yes, the editor could "Insert" the shot of the hotel (that is, cut it in and out) but the correct labeling of the shot would be a Cutaway because it is naturally outside the frame of the two people talking.
The importance of either an insert or a cutaway to the scene is a creative decision between Editor and Director but the correct labeling of them is important to identifying their use.
Now, let's take on the task of defining B-Roll. Not to be confused with the film terms A-wind or B-wind (describing whether the film comes off the core with emulsion in or out or for single perf film, whether sprocket holes are on left or right) B-Roll is a term most commonly used to label footage that will illustrate something rather than be the main or A subject. In a sense, B-Roll is a video-only cutaway comprised of as many shots as you like.
It's a common term in TV News where a reporter will record a standup after a news event is over. That report (the A-Roll) will be 'covered' with the B-Roll or edited footage of the actual event. In the old days, before non-linear editing (particularly with film) the reporter's A Roll was essentially a continuous dialogue track with picture, designed to be played straight through for sound (often with no editing) but not to be seen except for the on-camera open or closing sign off. In between, the reporter could look down and read his notes like a VO knowing he would not be seen again until the end when he looked up to sign off. To make it fast and easy they could edit a seperate reel of picture-only (no worries about editing sync sound) to play simultaneously against the sync of the reporter's A-Roll. This process required two projectors running simultaneously during the news show either electronically or manually tied to start together so the picture on the B-Roll would line up accurately with the A or reporter's on camera sync. While an extravagant use of projectors it allowd filmed news reports to be edited and aired quickly—a more important issue when it comes to News.
So today, B-roll is really a generic term meaning a seperate picture track, or footage shot to be used in that way. The technique is common today in non-linear editing when an editor uses the main track on his timeline for the main story/dialogue and builds a second track for visuals, whatever they may be or wherever they may be used, with our without their own sync sound. B-Roll is an easy way to organize footage that is illustrative or represents a different time or place such as a flashback. B-Roll can really be almost any image—either used to illustrate or as counterpoint.
Having such a broad meaning, the term B-roll lends itself to overuse. But now you know where it comes from and what it means.