Plagiarism occurs when you use someone else's work, ideas, or words without proper attribution. It is considered self-plagiarism when you reuse your own work without acknowledging that it has been previously published or presented elsewhere. Self-plagiarism can be problematic in academic and professional settings where originality and proper citation are expected. It's important to always give credit where it's due, whether it's someone else's work or your own, to maintain academic integrity and ethical standards.