Paraphrasing is one way to discuss or analyze another person's ideas without directly quoting them. Extensive paraphrasing, however, can lead to unintentional plagiarism. According to "The MLA Handbook for Writers of Research Papers," paraphrasing, or rewording someone else's writing, can be a form of plagiarism. When you condense a writer's argument into just a few sentences, for example, you may not be directly quoting, but you must still acknowledge the source of the argument. For that reason, paraphrasing large amounts of information is scarcely different from extensive quoting and must be properly cited as such.
Using quotation marks in your paper is not enough by itself -- you must also tell the reader exactly where the information came from. In-text citations are crucial for avoiding plagiarism. Doing so tells the readers who wrote the material and where they may find the information for themselves. In Modern Language Association of America (MLA) style, for example, you use a parenthetical citation at the end of a sentence to denote the origin of the quote. You may also make direct reference to an author in the text, acknowledging the author as your source of information. When you don't utilize in-text citations, your quotations may be considered plagiarism.
If you rely too much on another author's argument, you may commit plagiarism by over-quoting. While your teacher may simply consider this sub-par writing, he may also consider it plagiarism if you neglect to provide a sufficient amount of analysis or original argument. For example, the Purdue Online Writing Lab recommends quoting as little material as you need. Only use direct quotations when the author has phrased something better than you can or when paraphrasing would weaken the argument. Avoid block quotations, which convey a lot of information but also indicate large gaps in original writing or research.
Citing your quoted material in the text is not enough to avoid charges of plagiarism -- you must also cite your sources at the end of the paper. In MLA style, this comes in the form of a Works Cited page, in which you list every source that you used throughout the paper. A works-cited citation includes the author, title, place of publication, date of publication and the pages you used. Presenting such a citation acknowledges the sources for even your longest quotes.