Quoting significant passages from a source requires that a writer be on top of several requirements and details. In fact it actually includes, to some degree, use of the other two methods for incorporating others' ideas and words, summarizing and paraphrasing. They can, of course, also be used separately at different times, as can a direct snippet quote.
So one needs to understand what is different about these three techniques.
Summarizing means to provide the gist of a piece of writing. It's much shorter than the original and covers the main ideas only. An entire article, story, play, or novel might be summarized, or just a smaller component part of it.
Paraphrasing is a sort of translation into one's own terms what someone else has said. In using sources from earlier eras in the English language, it may well include real translation. It is also restating the information another way.
Summarizing requires restraint and a representation of the big picture. Articles in professional journals begin with an "abstract" that corresponds directly to the summary. It provides readers with just enough information to know whether this particular research report or article provides information relevant to the reader's interests and professional area of expertise.
Paraphrasing requires a responsibility to fairly represent the source material without leaving out anything or confusing the information. It is approximately the same length as the original it restates.
Here is a summary of the short story, "Charles," by Shirley Jackson.
The narrator only slowly grasps that her son, newly started in kindergarten, has been having trouble adjusting to the changes in his life, and that he is dealing with his stress by misbehaving in school. His creation of an imaginary friend, named Charles, provides him with a way to explain what has happened at school so that his parents don't punish him. Various clues along the way show the reader what is happening more clearly than the narrator sees it, and only in the ironic ending does she suddenly realize that Laurie is the child who has been causing so much trouble in his class.
Clearly, this is an overview of the story's thrust and focus. Nearly all the specifics of the narrative are missing, but the main idea has been clearly explained.
Here is a paraphrase of a short passage from the story.
First the original:
"The day my son Laurie started kindergarten he renounced corduroy overalls with bibs and began wearing blue jeans with a belt; I watched him go off the first morning with the older girl next door, seeing clearly that an era of my life was ended, my sweet-voiced nursery-school tot replaced by a long-trousered, swaggering character who forgot to stop at the corner and wave good-bye to me."
A paraphrased representation of this passage:
On his first day of school, Laurie refused to wear his formerly typical outfit of corduroy overalls and insisted on denim pants and a belt. He went off happily to school with the girl from next door, and I realized then that his babyhood was over: the little boy who spoke sweetly had become a big kid who insisted on different clothes and acted confident, even a little cocky. He didn't even even stop to wave goodbye to me when he got to the corner.
Note the important differences between the original and the paraphrased version:
1. Nearly all important words--nouns, verbs, adjectives, and adverbs--have been changed.
2. The information is provided in a similar order but the parts of speech are not in the same order. Many students believe that to paraphrase is to pull one word, insert a synonym, repeat a few more times, and that's it. No, the syntax of the sentences must not be the same. That qualifies as plagiarism.
3. The punctuation, the divisions of each sentences, can vary from the original, as long as all the information shows up in the paraphrase.
For more information on summary skills, see this site.
For more on paraphrasing, go here.