The failure to do this is plagiarism. In general when writing papers for college classes quotes and paraphrases are recommended only as ways of presenting another's argument as a point of reference in the process of making your own argument. In most essays and term papers you will be asked to do some original thinking, synthesizing materials from a number of sources and incorporating your own ideas. You should not use a paraphrase or a quote to present your own point. The plagiarist is the academic counterpart of the bank embezzler and of the manufacturer who mislabels a product.
Plagiarism takes many forms.
A second form plagiarism often takes is a string of directly borrowed words and phrases patched together into a new paragraph. In the resulting mosaic of other people's ideas and words, the writer's sole contribution is the cement to hold the pieces together. At the other end of the spectrum is plagiarism involving the casual use of a particularly apt word or phrase without referencing it. The examples given below should make clear the dishonest and the proper use of source material. If you are unsure in a particular instance whether or not to reference something, play it safe and acknowledge your sources.