Paraphrasing is often thought to be similar to summarizing, but rather than being simply a more concise version of a passage, paraphrasing requires you to restate all of the information from the original source and not just the main idea. Paraphrasing is more than just choosing a few synonyms and inserting them into the original sentence (this is considered to be plagiarism). You must also alter the syntax (the word and phrase order) of the sentence.
For example, if the original text is written:
Just above the northwestern shore of the old island of Hispaniola—the Santo Domingo of our day—and separated from it only by a narrow channel of some five or six miles in width, lies a queer little hunch of an island, known, because of a distant resemblance to that animal, as the Tortuga de Mar, or sea turtle.
Then this would be an example of plagiarism of that text:
Just above the northwestern coast of the ancient island of Hispaniola—the Santo Domingo of today—and divided from it by a thin channel of some five or six miles in width, exists a peculiar little hunch of an island, known, due to a remote likeness to that animal, as the Tortuga de Mar, or sea turtle (Pyle 3).
Why is this plagiarism?
Because the writer simply plugged in synonyms (the bold terms) without changing the structure of the sentence.
An appropriate way to paraphrase this text might be:
Near what we call Santo Domingo today, and what was called Hispaniola centuries ago, you can find the island called Tortuga de Mar, or sea turtle. This island is only five or six miles away from the northwest coast of Santo Domingo and got its name because it looks a little bit like a sea turtle (Pyle 3).