Vehicular Ad Hoc Networks (VANETs) have emerged as a new powerful technology with an aim of providing safety for the persons sitting in the vehicles. Vehicles may be connected to the Internet with/without the existing infrastructure using various IEEE standards such as IEEE 802.11p. But as nodes in VANETs have very high mobility, so there are lots of challenges to route the packets to their final destination which need to be addressed by existing/proposing new solutions for the same. Keeping in view of the above, this paper provides a detailed description of various existing routing techniques in literature with an aim of selecting a particular strategy depending upon its applicability in a particular application. A detailed categorization of various routing techniques is provided in the paper with critical discussion on each categorization with respect to its advantages, disadvantages, various constraints and applications. Finally, numbers of parameters are selected for comparison and analysis of all the existing routing schemes in the literature.