When you want to use the ideas and words of others to strengthen the points that you are making in your own writing, you must indicate this to your reader. Referencing appropriately whenever you use someone else's work to support yours.
Plagiarism is when you present someone else's work or ideas as your own, and it can be accidental or intentional. Students, academics and business people who are caught plagiarizing, face serious consequences such as failure, dismissal and legal action.
Plagiarism can be:
Watch this video from York St. John University for more information about plagiarism.
Take a look at the webpage Integrating Sources into My Paper for more guidance.