Plagiarists may steal content from several works, fine-tuning the phrasing to blend the stolen passages into a coherent whole and to attempt to disguise their plagiarism. Plagiarism also occurs when an author recycles substantial portions of their own previous writings into a new work, without presenting any original ideas or research.
While different institutions have varying definitions of what constitutes common knowledge, the essential qualifiers are the frequency with which the material appears in other sources without a citation and how many people are likely to already know it. When in doubt, cite your source because an unnecessary citation is preferable to a plagiarism charge.
Students who plagiarize risk failing grades and expulsion, while plagiarism in a professional setting can result in termination. If a plagiarism suit is brought to court, a ruling against a plagiarist can result in damage awards that can cost substantial sums of money.