This is not to say that you can’t use information that exists in other places.
A word to the wise:
Changing every third word in a passage does not count as paraphrasing. You are better off quoting and citing the actual source!
To avoid plagiarism, writers must rewrite the ideas in their own words (called paraphrasing) and cite the original source of the information.
Paraphrasing is similar to summarizing. Summaries only include the essential ideas of a work, while paraphrases include more details. Regardless of what you call it, you must cite the original source.
Depending on the circumstance, you may prefer to use an exact quote, surrounded by quotation marks. This is also acceptable, provided you give a full citation.