If you fail to properly credit ideas or materials taken from someone else, it is considered plagiarism.
To avoid plagiarism
A direct quote (verbatim, word-for-word) should follow a specified format.
Paraphrasing is a restatement of the original quotation.
There are some instances where a citation is not required.
A number of sources are available to help you decide if you have mastered the ability to paraphrase:
University of Central Florida, The University Writing Center, Plagiarism & Misuse of Sources http://uwc.cah.ucf.edu/wp-content/uploads/sites/9/2015/04/Plagiarism_and_Misuse.pdf
Purdue, Online Writing Lab, Is it Plagiarism Yet? https://owl.english.purdue.edu/owl/resource/589/02/
Indiana University-Bloomington, School of Education, How to Recognize Plagiarism; http://www.indiana.edu/~istd/example1paraphrasing.html