Plagiarism is the use of another's original words or ideas as though they were your own. If you do not give credit where it is due, you have committed plagiarism.
"A study by The Center for Academic Integrity found that almost 80% of college students admit to cheating at least once." Plagiarism.org
Plagiarism in the information age is not always a cut and dry issue. This site has frequently asked questions about plagiarism and its consequences. A caveat of using plagiarism.org is that it appears to be funded by the commercial business turnitin.com, but there is helpful, correct information.
In 2006, Eric Poehlman was the first researcher sentenced to prison for scientific misconduct. He falsified data. This is not direct plagairism, it is unethical conduct.
Historian Stephen Ambrose was nailed for plagarism in his book, The Wild Blue.