This book originated from an elective course called Internet and Classification Methods for master and doctoral students, which has been taught since the academic year 2013/14 at the Charles University and the Czech Technical University in Prague. The course is intended for students of the study branches Computer Science (Charles University) and Information Technology (Czech Technical University) and its main purpose is to make the students aware of the fact that a key functionality of several very important Internet applications is actually the functionality of a classifier. That functionality is explained in sufficient detail to remove any magic from it and to allow competent assessment and competent tuning of such applications with respect to that functionality. We expect, and the first years of teaching the course confirm it, that this topic is particularly interesting for those who would like to develop or to improve such applications. The Internet applications we consider are: