Intrusion Detection System (IDS) can be defining as a system which detects intrusive activities. In this paper, the authors discuss about intrusion detection system with their basic terminology. It also focuses on services and characteristics of detection system. The main function of IDS is to distinguishing normal and abnormal pattern of input data. An IDS is a software or hardware device that deals with attacks by collecting information from a variety of system and network sources and then analyze security problems.