Plagiarism is using someone else's words, ideas, or work and claiming that it is your own or failing to give them proper credit. The chart on this page explains the variety of plagiarism, which includes intentional, self-plagiarism, and unintentional.
Intentional Plagiarism: Taking credit for someone's work, when you understand that what you're doing is wrong
Self-Plagiarism: Reusing your own work in a different context
Unintentional Plagiarisim: Not giving someone else credit for their work, even if you didn't understand that what you're doing is wrong