Opportunistic routing can largely improve the performance of wireless multihop networks by taking advantage of the broadcasting nature of wireless medium. In this paper, the basic idea behind opportunistic routing is introduced, and then the paper looks to classify existing work in this area based on different criteria. A comprehensive survey on typical opportunistic routing protocols is given. This paper introduces, in details, how each of these protocols work, and then the paper discusses about their merits and drawbacks. Finally, this paper concludes with some issues that still need to be resolved in this area in hopes of stimulating future research on this topic.