There are different kinds of "plagiarism." Haxham (his site is mentioned at the bottom of this page) tentatively defines "Plagiarism" and "Academic Plagiarism" as such:
Plagiarism is the deliberate attempt to deceive the reader through the appropriation and representation as one's own the work and words of others. Academic plagiarism occurs when a writer repeatedly uses more than four words from a printed source without the use of quotation marks and a precise reference to the original source in a work presented as the author's own research and scholarship.
We may also propose the following:
1. Deliberate plagiarism: using someone's words or ideas without clearly acknowledging the source of that information.
2. Self-plagiarism: is when you take a term paper or essay that was written for one class and submit substantial parts or the whole paper for credit in a another class, without informing the instructor.
3. Accidental Plagiarism: occurs, although you don't intend to plagiarize, when you fail to cite your sources completely and correctly.
Use your own words and ideas
Using synonyms (e.g.: using “less” for “fewer”)
Reversing the order of a sentence
Common knowledge: Abraham Lincoln was the U.S. President during the Civil War.
Not common knowledge: There were 51,000 casualties at the Battle of Gettysburg.
Common knowledge: Joseph Stalin was born in Georgia.
Not common knowledge: Stalin was poisoned*.
*Jonathan Brent, Vladimir Naumov. Stalin's Last Crime : The Plot Against the Jewish Doctors, 1948-1953. HarperCollins, 2003