If you do not adequately acknowledge the source of your information in your essay and/or dissertation, this is termed as plagiarism. Please note that even if you give a proper citation in the text and a reference at the end of your essay/dissertation, you can be accused of plagiarism if you have copied word by word or sometimes even if you have paraphrased too closely. You must therefore write the ideas/arguments in your own words and duly provide a reference or use the inverted comas (” “) to quote (in which case you can copy word by word, however the quotations should not be more than 15% of the total number of words of your essay/dissertation).
A guide on Plagiarism by the University of Oregon
Please also note that there are many software systems that are used by the universities to detect plagiarism.You can use this software yourself to make sure that your work is completely original and non-plagiarised.