Why is plagiarism wrong?

Plagiarism is wrong for the following reasons:

It is dishonest, because it involves the theft of another person's information or ideas.
It is unfair to other students, because their work is graded against dishonest work.
It is disrespectful to tutors and lecturers, because it is a betrayal of their trust.
It is harmful to the reputation of the University and the qualifications it awards.

Plagiarism is wrong whether it is intentional or unintentional.

Intentional plagiarism is the passing off of another person's work as your own with the intention of deceiving your reader.
Unintentional plagiarism is the failure to acknowledge another person's words, ideas or data because of a lack of either understanding of plagiarism or skills in referencing. Unintentional plagiarism can also result from poor skills in paraphrasing.
Collusion is helping or joining with someone else in the act of plagiarism.