Among the various forms of malware, Botnet is the serious threat which occurs commonly in today’s cyber attacks and cyber crimes. Botnet are designed to perform predefined functions in an automated fashion, where these malicious activities ranges from online searching of data, accessing lists, moving files sharing channel information to DDoS attacks against critical targets, phishing, click fraud etc. Existence of command and control(C&C) infrastructure makes the functioning of Botnet unique; in turn throws challenges in the mitigation of Botnet attacks. Hence Botnet detection has been an interesting research topic related to cyber-threat and cyber-crime prevention in network security. Various types of techniques and approaches have been proposed for detection, mitigation and preventation to Botnet attack. Here I discusses in detail about Botnet and related research including Botnet evolution, life-cycle, command and control models, communication protocols, Botnet detection, and Botnet mitigation mechanism etc. Also an overview of research on Botnets which describe the possible attacks performed by various types of Botnet communication technologies in future. KEYWORDS— Bot; Botnet; C&C mechanism; communication protocols; honeynet; passive traffic; attacks; defense; preventaation; mitigation