Most academic writing builds on the ideas and writing of others. We reference other texts to help build arguments, to establish areas of consensus, and to identify claims with which we disagree or hope to build on. Most academic research papers have a "literature review" section that sets out to trace what has been thought and said on a topic, and which require authors to clearly identify the source of quotations used.
To do this responsibly, so that it is clear who said what, and to help readers trace the source of ideas and expressions, as well as images and other media, professional organizations (such as MLA and APA and AMA) have developed citation systems that help standardize how to give credit when we reference the texts of others. This guide will help you understand and avoid plagiarism in the context of a university setting, as well as understand the difference between the ethical offense that is plagiarism, and the legal offense that is copyright infringement.