Plagiarism refers to incorporating someone else’s work into your own writing in a way that’s identical or substantially similar to the original material without an appropriate acknowledgement. This means that copying a passage verbatim without putting it inside quotation marks is plagiarism; not citing your sourcing or citing them incorrectly also count as such.
Plagiarism can also be an infringement of copyright although the two are not coterminous (plagiarism is about ethics while copyright infringement is a legal term – you can read more about them at https://writ.rs/ copyrightvsplagiarism. Copyright applies to many different kinds of creative works, such as written text and images, regardless of whether they are officially published or not, or whether they are printed or appear online. There is a common misconception that you can just copy, paste and use anything that’s online, but this isn’t the case.