Datacenters and Cloud computing technologies are now in their growth stage with the generalization of Information Technologies. As useful as they may seem, their operation causes many problems among which energy consumption is one of the most worrying. Several techniques have been developed to reduce energy consumption in Datacenter environments. They differs by the methods used to analyze consumer trends, energy reduction mechanisms as well as assessment tools. Their results depend on the study contexts and are generally adapted to similar environments. In this work, we aim to present a state of art and a classification of the most important techniques of energy reduction in the virtualized environment of the Cloud. This classification, the first one at our knowledge, represent a basic framework for others enhancements of energy efficiency methods in Datacenters.