Plagiarism occurs when one person uses the work of another person without adequately citing the source. Although the most common example is someone directly copying sentences or paragraphs from an article or literary work, other forms of plagiarism can be more subtle.
Ideas can also be plagiarized. Although some things are common knowledge and do not need citations, many unusual or lesser-known facts do need to be cited, even if the author summarizes them in her own words. This is especially true in academic writing. To be safe, writers should generally cite any facts or theories that they learned during their research.
Paraphrasing can be a subtle form of plagiarism. While it is acceptable, there must be significant differences between the paraphrase and the source material. People who just replace a few words or make slight changes but mostly copy the original text verbatim are guilty of plagiarism.
Direct quotes are not plagiarism as long as they are properly set apart in quotation marks and cited. However, if the author overuses quotes to the point that the quoted passages make up the bulk of the paper, it may still be considered plagiarism.
Videos, music and images can also be plagiarized. This generally occurs when someone uses the material without proper attribution or permission. Creating an exact copy of another work may also be plagiarism, but it depends on the situation and whether the original work is cited.