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.
This is a tutorial from Golden Gate University's University Library on the importance of citing your work and how to avoid plagarizing.
Where to Go For More Information on Plagiarism and How to Avoid It
Your professors want to see your work--not someone else's. This is your grade, so it needs to be your work.
With Serious Consequences
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.