Copyright Infringement by 
Don't Steal Copyrighted Stuff! by 
Public Domain by “A legal device that provides the creator of a work of art or literature, or a work that conveys information or ideas, the right to control how the work is used." Stephen Fishman, Esq. The Copyright Handbook, 1996.
“Your work is under copyright protection the moment it is created and fixed in a tangible form that it is perceptible either directly or with the aid of a machine or device.” U.S. Copyright Office, 2006.
"Margaret Stewart, YouTube's head of user experience, talks about how the ubiquitous video site works with copyright holders and creators to foster (at the best of times) a creative ecosystem where everybody wins."
Another perspective from Lawrence Lessig, Harvard professor, and one of our foremost experts on copyright issues.
You may use up to 10% of the total, or 1,000 words, whichever is less.
You may use an entire poem of less than 250 words, but no more than three poems by one poet, or five poems by different authors in an anthology. For poems exceeding 250 words, you may not use more than three excerpts from one poet or five excerpts from different poets in the same work.
You may use up to 10% of the work, but no more than 30 seconds of the music or lyrics from an individual music work.
You may use no more than five images from one artist or photographer.
You may use no more than 10%, or 15 images, whichever is less, from a collection.
You may use up to 10% of the total, or three minutes, whichever is less.