Fair Use
There is a doctrine in the United States copyright law called “Fair Use” which allows people to use your content without your permission. Fair Use is now widely accepted in most countries around the world. It allows the limited use of copyrighted material without requiring permission from the copyright owner. Items considered Fair Use would be commentary, criticism, news reporting, research, teaching or scholarship. It provides for the legal, non-licensed citation or incorporation of copyrighted material in another author’s work under a four-factor balancing test:
the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
the nature of the copyrighted work;
the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
the effect of the use upon the potential market for or value of the copyrighted work.
This is important because if a smaller photo of yours is used or a few words out of an entire document is used this could fall under Fair Use depending on the site and the use. If someone were to take a paragraph of your work, or a site has copied your entire site or pictures, and they run a business profiting from your copyrighted work, then most likely its copyright infringement. It is usually very obvious when you see how your work is being used whether the content is being used under the Fair Use criteria or for profit.
You can find more on Fair Use here:
Other FAQ's that will interest you:
What do I need to know in order to create a Takedown case with DMCA.com?
Where do I go to create a Full Service Takedown case?
How do I sign up so I can do Takedowns on my own?