Plagiarism can have serious repercussions, including being kicked out of school. Unfortunately, many students do not have a clear idea of what constitutes plagiarism so continuing high school practices of copying and pasting from webpages gets a lot of students in trouble.
There are many things that can be considered plagiarism, but it boils down to the idea that if you did not write it, you are cheating. A best practices policy is to read everything you can find about your topic and take thorough notes, keeping track of the sources as you go. When you are ready to write, sit down and write. Everything you write, without using your notes, is your work. After you are done, go back into the notes to confirm details, such as numbers or dates. Anything you write with the notes in front of you needs a citation. There is another reading on how to decide what to put a footnote on, but the final answer is pretty much everything.
A useful tool for understanding plagiarism is at plagiarism.org. Pay particular attention to the six types of plagiarism found there in the article called What is Plagiarism?:
 “What is Plagiarism,” Plagiarism, accessed May 2, 2020, https://plagiarism.org/article/what-is-plagiarism.
Some types of plagiarism are easier to understand than others. Turning in someone else’s work as your own, for example, is clear. Others are less obvious about what is included. Changing words, for example.
Students often attempt to use a thesaurus to change words in something they are trying to copy and paste instead of just doing the work themselves. This rarely works. The type of student who does this is also the type of student who has not excelled in school and therefore does not have a strong vocabulary. They have a tendency to just pick any word out of the thesaurus without considering the nuances of the different words.
As an example, look up the word HOUSE in the thesaurus. You will see family or community. Probably not things you would use to describe somewhere people live. One step further and you will find words like abode, dwelling, building. None of these convey the same ideas. There are also many types of houses, such as castle, mansion, hovel, hut, shack, estate. It really stands out when you select any word that does not send the same message as the original word.
While simply changing words to mask the origin is straight-up cheating, paraphrasing is not quite so simple to identify. Some examples of the difference between paraphrasing and plagiarism can be found at this Rochester Institute of Technology site.
Another one you should notice is “giving incorrect information about the source of a quotation.” It is important you give accurate information about your sources. If you put the wrong source, it implies you are attempting to hide something. That can be anything from the idea that you really used a tertiary source to blatant falsification of information. Your citations must go where you say they will, i.e., links must work, and the information you say you got from there must be there. That is one reason including page numbers in the footnote is so important.
The last one to notice is “Failing to put a quotation in quotation marks.” Using quotes, and quotation marks, on anything except primary sources indicates you are letting the original authors do the work for you. This is similar to the point “Copying so many words or ideas from a source that it makes up the majority of your work.” Most professors will include the idea that quotes cannot be counted in the word count in order to ensure you do the work yourself.
What it really comes down to is that your work should be your own. You should read enough secondary sources to understand the subject well enough to write it in your words. Use primary sources to supplement that information in impactful and meaningful ways that support your points.