Intellectual Property on the Web
Digital files, like text and images on the Web, are easy to reproduce and edit. However most content found on websites is copyrighted by law and cannot be reproduced without an explicit permission from its author. Copyrighted content includes most creative property found on the web, such as text, images, video, audio and code. If you do not know if material on the web is copyrighted it is safe to assume that it is. Copyright laws apply regardless if the website displays a copyright notice. If you want to use materials created by somebody else, your best route would be to ask for permission from the author.
What is copyright?
According to the US Copyright Office, “Copyright is a form of protection provided by the laws of the United States … to the authors of “original works of authorship,” including literary, dramatic, musical, artistic, and certain other intellectual works. This protection is available to both published and unpublished works.”
Copyrights laws give author exclusive rights to reproduce, distribute, and display their work in public, as well as create other work derived from it. Authors can grant other people permission to reproduce their work or use it in other ways.
More about copyright laws.
Under certain circumstances it may be legal to make very limited use of copyrighted material without the permission of the author. For example, in a book review, you may quote a paragraph from the book you are reviewing, or in a article about a movie you may include an image from the movie. You still must attribute the work to its creator. Fair use usually implies that your intent is non-commercial, educational or nonprofit, that you use a small portion of the original material and that your use of it does not devalue its original source. However, Fair Use does not stipulate the amount of text, code, video or music that is permissible to use and your safest bet is always to seek author permission.
More about Fair Use.
Some authors choose to allow others the use their work on certain conditions. Authors using one of the Creative Commons licenses keep their copyright but allow people to copy and distribute their work on the conditions specified in the licence, and provided they give them credit. For example, in flickr.com/creativecommons you can find images published under several Creative Commons licenses. If you wish to use one of these images in your work you will have to carefully read the license of the image and comply by its rules.
More about Creative Commons.