To use any material that you did not create, it is important to do BOTH of the following:
1) Make sure you are permitted to use the material.
This can be done in any of these ways:
2) Give attribution for the material.
If you did not create it, you must cite it, regardless of how you obtained the permission to use the material. Even material in the public domain must be cited; failure to do so is unintentional plagiarism, as it implies that you created the content yourself.
Here's a really good introduction to copyright in plain English:
To access the Seton Hill University Campus Copyright Policies:
Many works available on the web will have Creative Commons licenses. This type of license allows the item's creator to reserve some types of use while allowing others. This website will explain the different types of permissions creators can set for their works and how to tell what you may do with a file that is made available through a Creative Commons license.