This paper provides an overview on the most important types of botnets in terms of network topology, functional principle as well as a short definition on the subject matter. By exploring the motivation of botnet operators, the reader will gain more insight into business models and course of actions of key players in the field. Furthermore, essential botnet modules, major important roles and infection vectors will be discussed in order to provide an overview. This paper will also treat the hiding, detection, as well as the decommissioning of botnets. Moreover, it will be discussed in what sense botnets may be considered as resilient systems and what estimations about countermeasures can be taken in order to tackle future developments in the field of botnets.