The exception to this rule is information termed general knowledge, information that is widely known and stated in a number of sources. Determining what is general knowledge can be complicated, so it is wise to remember the adage, "When in doubt, cite." In academic work, credit should generally be given in an appropriate format, for example, the systems created by the Modern Language Association (MLA) or the American Psychological Association (APA). These systems of documentation are explained in various research textbooks. (Occasionally, in writing, assignments such as log entries, professors may permit informal citation.) 

It is very important to understand how to prevent committing plagiarism when using material from a source. If you wish to quote verbatim, you must use the exact words and punctuation of the original and you must include quotation marks and page numbers in your citation. In general, using large amounts of quoted text is not appropriate.  When you quote large amounts of text you lose ownership of the paper. 

If you want to paraphrase ideas from a source, you must do a thorough job of putting the ideas into your own language and you must cite the source in the required format. 
 

The explanation above is adapted, in part, from  the English Department's definition of plagiarism.