Last week, we looked at what fair use ISN'T. So now let's look at what it is. Basically, fair use depends on a few factors, most importantly
(1) the amount of material you copy from the original work, (2) your purpose in using that material, (3) the nature of the work that you're copying, and (4) the effect that your copying has on the original art. (5) The amount of material you copy from the original work
For the first factor, you are generally allowed to use snippets of a work as long as you don't appropriate the "heart and soul" of the work...what, exactly, that means is open to some debate. As an example, if you grab a few seconds from another film, you're probably in the clear. However, you would probably not get away with including the entirety of that film in your own project. How much is too much? That's different in every case, so just try to exercise some common sense.
(2) Your purpose in using that material -- Fair Use allows you to make use of a pre-existing artistic work for purposes of education, training, news reporting, scientific research, critique or commentary, or parody. Think of the last time you watched critic review a movie on television. It probably included a short clip from the movie being reviewed, right? Likewise, you might have been in a classroom lecture where the teacher will show video clips to illustrate some point or other. These are other examples of situations that are more likely to be deemed Fair Use.
(3) The nature of the work that you're copying -- Certain sorts of material are more likely to fall under the fair use exception for example, clips from non-fiction works. We generally agree that it's a good thing for the public to be well-informed on history, so would it be in the public interest for a historical video like, say, the Zapruder film of President John F. Kennedy's assassination to be blocked from public view because someone claimed copyright ownership? (In fact, Time magazine did make such a claim after a history book used stills from the film as illustrations. They didn't prevail.)
(4) The effect that copying has on the original art -- Finally, let's look at the effect on the original work. Remember that copyright law exists in theory so that an artist can profit from his own hard work without someone else undermining him. Thus your work can't be so similar that people would start buying yours instead of the original. One sticking point for many is that fair use depends a lot on intentions and motivations. For example, let's pretend that I wrote a parody song making fun of Lady Gaga's latest song by changing the lyrics to be about cheeseburgers. It's fine if people listen to my song and think, Hey! This song makes a good point. Its sharp and biting satirical message has made me realize that Lady Gaga is a hack and I will henceforth refuse to purchase her music! If Lady Gaga's sales start to drop as a result, she can't sue me because my ridicule was too effective.
HOWEVER, I could get in trouble if people listen to my parody and think, Hey! This song is pretty good! It's pretty much like Lady Gaga, so instead of buying the original song from her, I'll just buy this parody instead. A parody work that hurts the original artist by becoming a substitute for their work will most likely not qualify for a fair use exception. Again, acknowledging that you don't own the original artistic work, giving credit to the original artist or using a work without any intention to profit will often encourage an original artist to let you use his work, but it does not necessarily mean that your work will fall under fair use.
If you really want to be sure you're in the clear when it comes to copyright with your video project, you owe it to yourself to check out Videomaker's Archival Storytelling book. It breaks down all the elements of copyright, so that you'll know exactly what you can get away with when you need to use footage from another person's work.