Claiming someone else's material as your own is plagiarism. This constitutes theft of material and, in certain instances, a breach of copyright.
Electronic plagiarism is a growing problem, particularly for educational establishments and exam boards. There are even websites which offer A-grade essays for sale and encourage students to submit their own high grade work for inclusion on the site.
Tools for detecting plagiarism, however, are becoming more sophisticated and those found guilty can be prosecuted.