There are limits to the exclusive use rights granted by a copyright. These limits are generally referred to as the 'fair use' of copyrighted materials. Fair use permits the use of copyrighted materials without the specific permission of the copyright owner. The distinction between fair use and infringement is often unclear and difficult to establish.
Federal law provides some guidelines to use in establishing fair use. Portions of a particular work may be reproduced and considered fair use if the purpose of doing so is criticism, comment, news reporting, teaching, scholarship, or research. So, including short passages of someone else's written work in your report or paper is allowed under the fair use provision. Don't forget the footnote! Showing portions of a film or listening to portions of someone's musical composition in class are also fair use of video or musical material.
If you subscribe to a service that provides legal copies of copyrighted work such as iTunes, you are required to use all downloaded materials (songs) in accordance with the service's rules of usage. Generally these rules prohibit you from sharing your downloaded files with others. You cannot make files you legally downloaded available for others to copy to their computers. Doing so is a violation of copyright laws and you will be subject to fines, legal actions and penalties.
There are guidelines for legally sharing someone else written materials, songs or videos. However, the guidelines are not clear or easy to apply to many cases.
The Best Rule of Thumb: When in doubt, get permission!
The CUSU system has more details about copyright at https://www.ct.edu/copyright