References, citations and avoiding plagiarism

"Plagiarism is the representation of other people’s work or ideas as your own without appropriate referencing or acknowledgement. This can include instances of copying from a book, journal or website without appropriate references and use of quotation marks, switching key words in a text with synonyms, paraphrasing without acknowledgement, incorrectly identifying quotations, or an incomplete bibliography."

See UCL's Academic Integrity webpage.

In your academic work you can be penalised for this form of academic dishonesty,

The producer of a work holds the copyright to the use, and reuse, of the work itself, no matter the form: photographers retain copyright of their images; musicians of their music; and poets of their poems.  Across all mediums the use of someone else’s, or even your own, work without properly crediting them constitutes plagiarism and breach of copyright. 

Copyright restricts what you may do lawfully with someone else's work. You may face legal issues for infringing someone's copyright.