If you quote or paraphrase another author's work without including a reference to it you are plagiarising. Not only is it very easy to detect plagiarism using online services like Turn It In, but it is also very easy for your tutor to spot it just by reading your work. Remember - you are not being marked on your ability to write facts or show off what you know. Any assumptions or facts you state must have someone else's credible work to back you up. Plagiarism does not only mean cheating, it is mainly used to describe forgetting or not realising to include a reference to other's work or theories.