You probably already know (or think you know) quite a bit about plagiarism, but sometimes what you learned in high school or in other classes is incomplete. In fact, frequently it's not quite accurate.
Plagiarism is the act of taking ideas, words, or other information from a source without indicating your use of that source. This is also known as not-giving credit to the source.
Using your own words to paraphrase or summarize from a source without giving credit to the source.
Copying whole sentences from a source without giving credit to the source.
Copying parts of sentences from a source without giving credit to the source.
Copying phrases from a source without giving credit to the source.
Copying diagrams, illustrations, charts, pictures, or other visual materials without giving credit to the source.
Copying media, including images, audio, video, or other media without giving credit to the source
By the way, to copy means to make a similar or identical version of. In your writing, it is NEVER appropriate to copy text from a source (even short phrases) and pass the text off as your own.
Some students think that anything they find on the web is free to use, and it's ok to copy it, and there's no need to give credit - but this is incorrect. Copying from the web is considered plagiarism.
The bottom line
Give credit for anything that originates somewhere outside of you (if it is not already considered common knowledge) - whether you are summarizing, paraphrasing, or quoting. When in doubt, give credit. It's safer.