This paper presents a novel approach to characterise applications with respect to their energy consumption by using a set of energy-related metrics, called Green Metrics. These indicators are based on energy consumption measurements, such as indices of computing resource usage, of environmental impact, of development costs required to (re)design an application in order to optimise its energy consumption footprint, and of organisational factors related to application management. Our approach is framed in the Green Active Management of Energy in IT Service Centres (GAMES) EU project, with a particular focus on green IT. In this paper, we define Green Metrics enabling to characterise an application in terms of the energy it consumes at run time. Such metrics are the basis for measuring the 'Greenness' of an application and to detect where it consumes and wastes energy. Hints are provided to improve applications design and execution. Using two application scenarios, we show how monitoring and evaluation of Green Metrics enables to improve energy efficiency.