The University of Cambridge has a clear definition of plagiarism.
In essence, any work that you submit should be your own original thought. If you use someone else’s ideas or argument you must acknowledge that. It doesn’t matter whether it is directly quoted or paraphrased in your own words, or whether the other person is an author or one of your friends.
To avoid plagiarism you should carefully follow your departmental/faculty guidelines and always acknowledge your sources. Make sure when you are writing notes that you can differentiate between quotations from works, summarising of works and your own thoughts about what you have read. Try colour-coding or symbols to make the distinction, so you still know when you come back to your notes later. (See also How do I cite and reference or How do I use referencing software.)
Ignorance of plagiarism is not a valid excuse, and the University places certain responsibilities on students regarding plagiarism If your department/faculty doesn’t seem to have guidelines online check course booklets, Moodle sites, or ask your Director of Studies or the library staff if you still can’t find the information you need.
If you’re interested in what guidance is available to academic staff and examiners you can find more information on the policy, procedure and guidance page.
