Vehicle-to-everything (V2X) communications represent information sharing between vehicles and any elements involved in an intelligent transportation system (ITS), including other nearby vehicles, handheld devices carried by pedestrians, roadside units (RSUs), and remote V2X application servers. At present, V2X technologies can be divided into two categories: IEEE 802.11p-based standards and LTE-based standards. The dedicated short-range communications (DSRC) in the U.S. and the ITS-G5 in Europe are two typical IEEE 802.11p-based systems, which reuse PHY and MAC of the IEEE 802.11p without or with small modifications in the bottom layers. Meanwhile, 3GPP are also developing LTE-V standards for supporting V2X services in cellular systems. In this paper, we first review two IEEE 802.11p-based ITS standards, including DSRC and ITS-G5. We then overview technical enhancements of the newly introduced LTE-V standard for V2X in 3GPP Release 14. In addition, advanced V2X applications are also described. Finally, we present several research challenges.