The obtainability of spectrum is limited for wireless communication. The inefficient usage of spectrum leads to the evolution of a new technology known as Cognitive Radio Networks. With the fixed spectrum assignment policy, most of the spectrum remains unused and remains as vacant white spaces all the time. This leads to an approach known as spectrum sensing. Spectrum sensing is the process of detecting the presence of the primary users (PUs) in a licensed spectrum and is considered as fundamental problem for cognitive radio. It is an intelligent radio communication approach where a cognitive radio device will sense the environment and carry communication using the unutilized spectrum bands. Cognitive radio technology potentially satisfies the increasing users in future. This paper provides an insight of cognitive radio knowledge- architecture, cognition cycle, functions, different spectrum sensing methods and various applications of CR.