Citing and referencing is about recording and acknowledging the sources you have used in your academic writing, in order to avoid plagiarism.

Acknowledging the information sources you have used in your writing is done by:

Citing the work you have used in the text of your writing, with brief details (In-text Citation)
Including the complete details of the source in a list of references at the end of the assignment (your Bibliography or List of References)

Any books, journal articles, newspaper articles, websites, images, data etc. you use in your assignment should be acknowledged because:

It gives credit to the person whose ideas you are using (thus avoiding plagiarism)
You get credit from your lecturers for doing your research
It positions your work within the existing body of knowledge

You must cite the source , if you are going to:

[Handout] Referencing key points & examples

Citing, Referencing & Plagiarism | Module Navigation

Section 2.3: Referencing Styles | Section 2.4: Citing and Referencing Tools | Conclusion