Plagiarism is the presentation of information (either written or oral) as one’s own when some or all of the information was derived from some other source.
From Manchester University's 2014-2015 Student Handbook
Paraphrasing is an ideal way to incorporate another author's ideas into your own paper without worrying about plagiarizing or using excessive quotations. Paraphrasing is not switching out a few words or rearranging a sentence but restating information in your own words. Paraphrasing is still borrowing another individual's ideas, so you do need to cite the original author.
To help make sure you're paraphrasing and not plagiarizing try this:
Sometimes an author has written something so clearly and concisely that it's not possible to paraphrase without creating nearly the same sentence(s). In that case, use a quote. Better safe than sorry!
If you'd like to practice your paraphrasing ability, try these exercises from the Purdue OWL: