In recent years, reducing energy consumption has become a popular research subject in different areas. Environmental issues such as Greenhouse gas emissions, global warming and energy cost are important factors which have led into many researchers do research in the field of green and energy aware computing. In the other hand, with the growth of new technologies and increase their popularity, considering the energy efficiency and green technologies is important. One of the new technologies that are today considered to be the area of cloud computing. Cloud computing is one of the new technologies which is very popular and widely accepted in the industry. The growth of cloud computing services may lead to consume a huge amount of energy and emit considerable amount of carbon dioxide. In this paper, a review of green cloud computing, common metrics fie measuring the greenness and energy efficiency, and finally the classification of works based on their level of approaches and used algorithms will be presented.