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.
Your professors want to see your work--not someone else's. This is your grade, so it needs to be your work.
At the very least you will not recieve credit for the work you turned it if it contained plagiarism. If your plagarism is severe and/or you do it multiple times you can be expelled from the university.