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Opportunistic networks, which are seen as one of the major evolutions from MANETs, have attracted many attentions recently. Generally speaking, opportunistic networks cover many similar aspects of Delay Tolerant Networks and Disruption Tolerant Network, which can be characterized by intermittent connectivity, frequent end-to-end path disruption and topology changes. In such challenging network scenarios, the end-to-end network resilience, or the effectiveness and efficiency of underlying network, becomes a very important topic. To improve network resilience in opportunistic networks, a new breed of forwarding technologies, namely opportunistic forwarding, has been proposed recently. However, many of such works are developed independently and thereby following fairly different terminologies. In this paper, (i) we first give an overview of opportunistic networks; (ii) we then address the applicability, definition, and taxonomy of opportunistic forwarding technologies; next, (iii) we provide insights on fundamental ideas behind opportunistic forwarding, while walking through a few representative examples; finally, (iv) we conclude the paper with discussion on issues and challenges.