I have recently heard some people haphazardly claiming that their use of other people's graphics and/or music in their game was ok due to "Fair Use".  Typical arguments:

"I'm not making a profit, so it's fair use."

"I'm only using a few images, so it's fair use."

My response to one of those individuals would be that even cursory research into how fair use works and previous fair use cases pretty quickly establishes that:

Fair Use is not as straightforward as they think.

Fair Use is complicated, I am not a lawyer, and this is not legal advice.  That being said, however, it is worth noting that there are a lot of resources available to help you learn more about Fair Use and to get a better idea if whether it really applies to your usage.  The Wikipedia article on fair use is a good start.  The wikipedia article makes it clear that, while the question of commercial/non-commercial use and how much of a work is being used are factors weighed by the courts, there are other factors that are weighed also.  Merely meeting one or two factors does not necessarily mean you are not infringing - Wikipedia specifically goes into detail on other factors such as whether the use is transformative or derivative in nature, and the effect of your use on the market for the original material.  Sure you may think your use of "Ice Ice Baby" in your incredible youtube game trailer doesn't directly compete with the song in the marketplace, but does it negatively impact the companies ability to get royalties for the song if anybody can simply use it for free?  Stuff like that matters.  

And ultimately, of course, it is very possible to win in court and still lose financially.  So tread carefully, do your homework, and if in doubt contact a lawyer.

Some further Info to get you started:

  Wikipedia fair use article

  A discussion on the risks of using a copyrighted song in your Youtube video

  Brief overview of Fair Use from Copyright.gov

Have questions about copyright licenses or how and where to find free graphics/sounds to use in your game and avoid concerns of "fair use" altogether?  My research on quality free sprites and sounds for use in game development is a good start!