Intrusion detection systems are software and/or hardware components that monitor computer systems and analyze events occurring in them for signs of intrusions. Due to widespread diversity and complexity of computer infrastructures, it is difficult to provide a completely secure computer system. Therefore, there are numerous security systems and intrusion detection systems that address different aspects of computer security. This paper first provides taxonomy of IDS with a simple description. Secondly a common architecture of Intrusion Detection System (IDS) and their basic characteristics are presented.