Study on optical correlation function initiates the development of many quantum techniques, with ghost imaging (GI) being one of the great achievements. Upon the first demonstration with entangled sources, the physics and improvements of GI attracted much interest. Among existing studies, GI with classical sources provoked debates and ideas to the most extent. Toward better understanding and practical applications of GI, fundamental theory, various designs of illumination patterns as well as reconstruction algorithms, demonstrations and field tests have been reported, with the topic of GI very much enriched. In this paper, we try to sketch the evolution of GI, focusing mainly on the basic idea, the properties and superiority, progress toward applications of GI with classical sources, and provide our discussion looking into the future.