The United States copyright laws are designed to prevent people from copying and distributing other people's work without permission. This includes both paper copies (i.e., photocopies, typewritten copies, etc.) and electronic copies (scanned or uploaded).
In an academic setting there is a fair use exception to the permission requirement, but it is only available if you meet the guidelines. If you do not meet the guidelines, permission must be sought.
1) Items in the Public Domain
Is the item in the public domain?
Copyright is not eternal and actually expires after a time. When copyright expires, the work enters into the public domain. For example: unpublished works (eg: your grandfather's old journals in your attic) enter the public domain 70 years after the author's death.
2) Fair Use
Can you use it under "fair-use" laws?
Using small pieces of a copyrighted work for parody, research/scholarship, or non-profit educational purposes is usually allowed under fair-use laws. Check out the sources below to find out if you are covered by "fair-use."
3) Used With Permission
Do you have permission to use the copyrighted item?
(Note: This includes the resources listed on the "Images, Music, Video" page)
You can write to the owner of the item and ask for permission to use it. Track down the owner though tools such as this.
Or, the owner may have willingly placed their content in the public domain. One way that owners can do that is through a "Creative Commons" license. Notice that the videos on this page have Creative Commons licenses.
Even if you have permission to use a copyrighted work you must still give attribution (give credit to) to the creator.