When you want to quote or paraphrase a section from a source for example a book, journal article or from an online source for your studies, you may be able to rely upon one of the "fair dealing" exceptions, in particular the Quotations exception. If you are reusing someone's work in a commercial context, such as a published book, it is less likely to be covered by an exception and therefore advisable to seek permission of the author or publisher.
If you copy or reproduce the work of others without crediting the author (referencing it), thereby giving the impression that it is your own work, you will be open to charges of plagiarism. Plagiarism is a serious academic issue, and can have serious consequences, however it is not a legal issue. Copyright is a matter of law and infringement may result in legal action. You infringe copyright and are at risk of plagiarism when you reuse someone's work without their permission and without crediting the author.