There are differences in opinion over how much credit must be given when preparing an article or historical account. Usually, reference is made to the original source as much as possible, and writers avoid taking credit for others' work.
The use of facts, rather than works of creative expression, does not constitute plagiarism. For example, it is illegal for a student to copy several paragraphs of text from a public domain book, and directly add the quotations to his or her own paper. If the quotations were not identified then they would be guilty of plagiarism, since they are using the work as it if were their own. High Schools colleges and universities are especially strict to the issue as each have their own academic code of ethics which prohibit plagiarism in all forms.
It is also considered plagiarism to take specifics from someone else's idea and present as their own work. A popular example being "Cliff Notes" which the students use to present their own analysis of the work they are reading. There is also a small market which has emerged offering essays and papers for sale which students can use, with a counter company emerging offering for instructors offering a database of source material they can search to see if their students paper has been plagiarized.
According to some ethic codes and criminal laws, a complaint of plagiarism can be proven by any person. The person complaining does not need to be the content owner of the material for their to be an investigation or discipline process initiated in response.
It is not plagiarism when two or more people come up with the same idea or analysis. This term is known as simultaneous inspiration (great minds think alike), and occurs as the result of the people being exposed to the same source material and interpreting it similarly.
Another term is accidental plagiarism. An example being a kid whose mother read him the same bedtime story, then later in life while writing a story for an assignment the story "came to him."
Due to fear of litigation, many editors refuse to recognize the difference between simultaneous or accidental inspiration and true plagiarism. Many universities will even revoke a degree if an alumnus' plagiarism comes to light within a year of graduation.
There are three types of Plagiarism:
Failing to cite quotations and borrowed ideas
Failing to enclose borrowed language in quotation marks
Failing to put summaries and paraphrases in your own words