Plagiarism is "the appropriation or imitation of the language, ideas, and thoughts of another author, and representation of them as one's original work" (The Random House College Dictionary, rev. ed.). It is theft.  

The University of Tennessee student handbook lists several examples of plagiarism.  Specific examples are: 
—copying without proper documentation (quotation marks and a citation) written or spoken words, phrases, or sentences from any source; 
—summarizing without proper documentation (usually a citation) ideas from another source (unless such information is recognized as common knowledge); 
—borrowing facts, statistics, graphs, pictorial representations, or phrases without acknowledging the source (unless such information is recognized as common knowledge); 
—collaborating on a graded assignment without instructor's approval; 
—submitting work, either in whole or part, created by a professional service and used without attribution (e.g., paper, speech, bibliography, or photograph).

Because plagiarism is serious does not mean that you cite everything to avoid the consequences.  The common rule of thumb is that if you can find information from three different sources, then it is common knowledge.  Common knowledge does not need to be cited.  Of course, things such as statistics are not common knowledge even if cited in three sources.  

If you are caught plagiarizing in my course, you will receive an F for the course and be referred to the appropriate university authority for further review.