There are four different types of plagiarism: direct, self, mosaic, and accidental. Direct implies that the writer knew they were copying. If you re-use the content of your own that has already been submitting, that is self-plagiarism. Mosaic is when someone takes bits and pieces of a bunch of different articles and pieces it together to make it seem unique. Accidental is just that, someone forgets to cite a reference correctly or give credit to the original author.