Plagiarism means copying someone else's text and forwarding it to the teacher as your own. Plagiarism is also copying parts of other authors and not citing them. This means that you steal somebody else's product. Plagiarism is forbidden also because teachers in such cases do not know if students have read and understood the materials.
Plagiarism is forbidden. If you write exam individually in the form of an essay you cannot answer a question by simply copying somebody else but you should prepare your own comparison or analysis. You can cite certain parts, especially written by famous authors but not whole paragraphs. The final product should be yours.
Plagiarism is:

• using sentences copied from a book and not citing the author
• copying texts from other essays, books, Internet etc.
• copying almost every word of some other author
• using statistical data and not citing the source
• copying numbers, graphs, photos, etc. without citing the source
• copying exams or seminar work of some other student.

You should avoid plagiarism by using your own way of thinking about ideas and information which you found in the study materials. Sometimes it is acceptable that you copy a sentence or paragraph if you wish to support your own opinion with something that was already stated by somebody else, especially if that one had more experience than you. In such cases you put down the surname of the author, the year of the publication and the page on which the reader can find the author. Then you describe such citation more in detail in the list of references at the end of the essay.