AliEn: ALICE Environment on the GRID

<!--HTML-->AliEn is the GRID middleware used by the ALICE collaboration. It provides all the components that are needed to manage the distributed resources. AliEn is used for all the computing workflows of the experiment: Montecarlo production, data replication and reconstruction and organixed or chaotic user analysis. Moreover, AliEn is also being used by other experiments like PANDA and CBM. The main components of AliEn are a centralized file and metadata catalogue, a job execution model and file replication model. These three components have been evolving over the last 10 years to make sure that the satisfy the computing requirements of the experiment, which keep increasing every year.