Mimicking biological ecosystems, digital ecosystems refer to complex and interdependent systems and their underlying infrastructures by which all constituents interact and exhibit as a whole self-organizing, scalable and sustainable behaviors. This emerging topic is of great interest to scholars. Nevertheless, we observe a divergence of opinions and a disagreement about what Digital Ecosystems are, how they are designed, and what their applications should be. In this paper, we present an overall view of the concept of Digital Ecosystem and provide background for a common understanding. Firstly, we identify four factors that have lead to the emergence of Digital Ecosystems and present their definitions, characteristics and categories. Secondly, we discuss models and simulation tools to show current achievements and limitations. Finally, we illustrate the main challenges and shed light on further work in the field of Digital Ecosystems.