Due to the fast growth in wireless communication services, the need for radio spectrum increased. However, most of the suitable radio spectrum has already been allocated using long term licenses. A considerable part of the allocated spectrum is underutilized over time and space. Cognitive Radio (CR) technology has arisen to solve the spectrum scarcity problem by allowing cognitive radio devices to opportunistically make use of the unused frequency bands in the allocated spectrum, which are termed white spaces or spectrum holes. Four CR functions have to be performed to allow CR devices to efficiently utilize the available spectrum holes without interfering with licensed devices already operating in the allocated spectrum. This paper presents a survey of the CR technology, its architecture and operation, a detailed description of the four CR functions, and the techniques and processes used in each function.