Disclaimer: not a lawyer, but based on the experience from usage (and making) of such tracks -
All music is "copyrighted" by default unless the creator explicitly has in writing abandon those rights. The author can also write it's OK to use his music for "anything". Copyright only means that the author has the right to determine the usage of his track. If the track is used commercially or not doesn't really matter, but commercial tracks are often restricted by default by an organization that represent the artist.
"Fair use" only applies to USA and UK. Though other countries has similar variants and various restrictions/liberties in relation to usage.
For the USA version of it: Fair use means you can use any copyrighted material with another purpose than public performing or sharing. More specifically you use an excerpt of the track to criticize it, review it, as an example in relation to education and so forth.
You can not just use it, or a part of it as a background piece or in some way which does not have a purpose as mentioned above (unless you have permission through for example royalty agreements with the organization or the artist).
Lets say you are a music journalist in a radio or TV station and doing a review of a track. Here you may use a fair part of the music, a part that is sufficient for the task so you can make your point but not so large you have in practice performed the "whole" track publicly.
If you work as a teacher in a lets say a music school and want to show the student how to make a certain effect, bridge, sound or whatever, you can use a part of the music sufficient to demonstrate what you try to teach, but you can not play the whole track or the irrelevant parts of the track for the students (this last would be considered public performance).
If you post a video on YouTube showing a screen recording of a game and use a few seconds of a track as background music - this is not fair use.
If you make a movie trailer and use a part of a cool track you like - this is not fair use (however, there are exceptions in some countries to this for radio/TV stations that makes in-house trailers (commercials for their own programs/shows), I am not sure about USA on this).
Bottom line is - Use common sense and not the heart and love for the track you want to use! :-)
If you want to use a track outside the fair use you can always become a member of the organization which manage the copyright and pay royalties for the use if it is public performance related. If you want to use it for non-generic performance (movie, commercial, youtube video..) you will need a written permission to use the track for that purpose (and it comes, most likely, with a price tag).
Worth to mention: there are commercial music made especially for usage in films, commercials etc. where you don't have to pay royalties and where you have automatically all permissions you need to use it in that way. There are plenty (too many to list here) of company sites where you can buy this from. For instance SmartSound, iStockphoto Music ++.
