To avoid plagiarism in your assignment or thesis, you need to supply references when you include quotes and paraphrased text from another source. In your writing you provide an in-text citation or footnote number directly after the phrase, sentence, or paragraph in question and a full reference in the references list or bibliography.
Not only text, also data, illustrations, charts, graphs, photos, videos, films, paintings and other art forms that are taken from another sources to illustrate your text should be referenced.
Common knowledge is not referenced, for example: Amsterdam is the capital of the Netherlands. What common knowledge is, may be subject to different opinions. Especially where subject-specific common knowledge is concerned it needs to be clear to your audience/readers that you are referring to something which is commonly agreed on, for example: In a computer, the external memory is the component where data is stored that is not in use by the processor.
In the exercise Is a reference necessary when...? you are presented with different situations. Select when a reference to a source is needed.