Why is plagiarism such a big issue? 

Plagiarism is an academic integrity violation, because it is considered ethically wrong to take credit for someone else’s intellectual output. This applies even if the original author agrees that you can use their output or if you paid to use it. And as you saw, it also applies to reusing your own work without citing it.

What is included in intellectual output?

Almost everything!  Written and spoken words, music and videos, movies, performances, artwork, photographs, graphs, tables, figures, diagrams, data, computer code, and any other intellectual or creative product.

It also extends to IDEAS, for example a specific method or model, the plot of a story, the outline of an experiment, and in your academic writing even the structure of a template or of paragraphs you borrow!

To avoid accidental plagiarism, you therefore ALWAYS need to give credit to the person whose work or idea you are using, and you do so by ACCURATELY quoting, PROPERLY paraphrasing and CORRECTLY citing your sources of information.