Plagiarism is using the work of others and pretending it is your own. The Merriam Webster Dictionary defines plagiarizing as "to steal and pass off (the ideas or words of another) as one's own" (emphasis mine).
It is plagiarism if you copy someone else's words without giving them credit.
It is plagiarism if you make only light cosmetic changes to someone else's words, such as using a synonym.
To test your understanding of Plagiarism, play the Goblin Threat Plagiarism Game.
This is a tutorial from Golden Gate University's University Library on the importance of citing your work and how to avoid plagarizing.