This guide, and Citation Style Guides, will help to familiarize you with details on what plagiarism is, and how to avoid it, and how to demonstrate academic integrity.
Oxford English Dictionary
Plagiarism, noun, The action or practice of taking someone else's work, idea, etc., and passing it off as one's own; literary theft.
The following, quoted from What is Plagiarism?, are examples of plagiarism (even if committed unintentionally):