Why is it important?
The person who has written the original source has the right to be acknowledged as the author for his or her own work. It is also important to clearly identify which parts of the text you have cited and which are your own thoughts and ideas. Referencing is even a way for you to display your knowledge of the scientific research in your field and for the reader to ensure you have interpreted this research correctly. Moreover, it is a way for you to show the reader what type of sources you have used, and it gives your reader the possibility to find this information for further reading if he or she is interested.
Through careful referencing you can avoid plagiarism.
A reference (or paraphrase) and a citation are different ways of expressing an idea or concept in the text that is not your own. A reference is a short summary of another's idea, theory, fact etc. that you present in your own words. The reference must include the author or copyright holder's name and the year the referenced source was published.
A citation is used when you wish to quote a text or speech verbatim. It is a direct copy of a phrase, a sentence, or a paragraph that you use in your own text. Just as with a reference, a citation requires the copyright holder's name and the year of the source's publication. The page number of the text where the citation can be found must also be provided.
Common knowledge is information that many people are aware of and is found in many sources. It does not require attribution of the source material, that is, you do not need to provide a citation.