IEEE 802.15.6 is the first international Wireless Body Area Network (WBAN) standard that supports communications in the vicinity of or inside a human body to serve a variety of medical and nonmedical applications. The standard defines a Medium Access Control (MAC) layer that supports several Physical (PHY) layers. In this paper, the key features of the IEEE 802.15.6 standard are presented. The MAC, PHY, and security specifications of the standard are explained in detail. Different communication modes and access mechanisms are presented. The Narrowband (NB), Ultra-wideband (UWB), and Human Body Communications (HBC) specifications are reviewed in terms of frame structure, modulation, and other important parameters. Finally, the security paradigm and services of the standard are highlighted.