In the age of information technology intellectual property becomes especially valuable. This is one of the causes why the cases of the plagiarism appear more frequently in all vital sectors. Due to that, there is a growing need for different instruments for the protection and verification of copyright for finding plagiarism. Before checking the document for plagiarism, reviewing algorithms and approaches for searching plagiarism, you must know and understand what constitutes the plagiarism. Therefore, in this paper we discuss definitions of plagiarism itself and give a look into most important types of plagiarism. As also the paper describes the most common plagiarism detection systems, methods used in those systems, and provides a description of several programs designed to compare documents and detect plagiarism.