Plagiarism is the improper use of another's ideas or words.
Two types of plagiarism exist: plagiarism of structure and plagiarism of content.
Plagiarism of structure is the use of another's logical order in an essay, steps of reasoning in a paragraph or section, or order of elements in a sentence.
If an author uses six paragraphs to describe in detail the process through which a dictator took power, you cannot construct your essay to copy that exact structure of six paragraphs detailing those steps. You can summarize those six steps within your work, as long as you provide a proper citation.
Additionally, if an author states, "Since the proletariat rose as a whole to support the dictator, they created the means by which he took control," and you change the sentence to say, "Because the great majority, made mostly of workers, got behind the dictator fully, they made the situation in which he came to power," you have paraphrased the author's idea but copied his original structure. Even with a citation, this is plagiarism because you have directly copied the author's structure. In this case, a direct quotation would have been appropriate.
Plagiarism of content is the failure to cite a summary, paraphrase, or direct quotation; failure to put a direct quoatation in quotation marks; and failure to keep summaries and paraphrases in your own words.
A summary is the condensed information from an original source. For example, a summary of a three-paragraph section may be condensed into one or two sentences. A summary should not repeat any specific words or phrases unless they are placed into quotation marks and cited separately. Summaries are always cited.
A paraphrase uses roughly the same amount of words as the original source. It should not repeat any specific words or phrases, and it should not order elements in the same fashion as the original (see plagiarism of structure). Paraphrases are always cited.
Any direct word-for-word use of any of the author's original language must be placed within quotation marks and properly cited.