I have put together various 802.11 definitions and standards from various sources available online and as always wikipedia had some detailed information on this context.
802.11 standards comprises of layer 1 (physical layer) and layer 2 (media access control) specifications of for implementing wireless local area network (WLAN) communication in the 900 MHz and 2.4, 3.6, 5, and 60 GHz frequency bands.
802.11 standards defines a medium of communication over the air between a wireless supplicant (Client device) and a station (Access Point) or between two capable wireless client devices. These standards were approved in 1997.
The 802.11 standards consists of a series of half-duplex over-the-air modulation techniques that use the same basic protocol
802.11 : This standard was released in 1997 by IEEE. This standard allowed wireless communication using 2.4 GHz bit rates of 1 and 2 Mbps. Frequency hopping spread spectrum (FRSS) and Direct sequence spread spectrum (DSSS) transmissions were using bit rate of 1 and 2 Mbps.
802.11a : The 802.11a standard was released in 1999, around the same time as 802.11b.It uses a 5 GHz frequency band and supports data transfer rates of 54 Mbps. 802.11a uses an orthogonal frequency division multiplexing (OFDM) encoding scheme. During those days there were fewer client devices with this capability and had lesser adoption.
802.11b : This Wi-Fi standard was developed by the IEEE in 1999 for transmitting data over a wireless network using the original standard of 802.11. It operates on a 2.4 GHz band and provides a raw data rate of 11 Mbps. As this standard evolved rom the original standards, lot of devices saw a jump in throughput and enhanced user experience with a wide acceptance of WLAN technologies.
Devices using 802.11b experience interference from other products operating in the 2.4 GHz band. Devices operating in the 2.4 GHz range include microwave ovens, Bluetooth devices, baby monitors, cordless telephones, and some amateur radio equipment.
802.11g : The 802.11g standard was released in 2003, this standard used OFDM technique and was able to achieve raw data rates of 54 Mbps compared with the 11 Mbps theoretical maximum with the earlier 802.11b standard. 802.11g operate at radio frequencies between 2.400 GHz and 2.4835 GHz.
802.11n : This standard was proposed in 2007, 802.11n is an amendment that improves upon the previous 802.11 a/g standards by adding multiple Input and Multiple Output antennas (MIMO). 802.11n operates on both the 2.4 GHz and the 5 GHz bands. It operates at a maximum data rate from 54 Mbps to 600 Mbps. This standard allowed channel bonding where we could combine two 20 MHz adjacent channels to achieve 40 MHz channel bonding which allowed more data to be transmitted. 40 MHz channel bonding is recommended on 5 GHz radio band only.
802.11ac: The 802.11ac standards were amended in 2013 and had major enhancements that was built upon previous 802.11 standards, particularly the 802.11n standard.
Changes compared to 802.11n include wider channels (80 or 160 MHz versus 40 MHz) in the 5 GHz band, more spatial streams (up to eight versus four), higher-order modulation (up to 256-QAM vs. 64-QAM), and the addition Multi User (MU-MIMO).
The 802.11ac standard operates only in the 5 GHz frequency range and features support for wider channels (80MHz and 160MHz) and beam forming capabilities by default to help achieve its higher wireless speeds.
802.11 ax: 802.11ax is the successor to 802.11ac standard, and is designed to increase the efficiency of WLAN networks. This standard is currently under developemnt is projected with the goal of providing 4x the throughput of 802.11ac.
802.11ad: 802.11ad is an amendment that was approved by IEEE in 2014, this standard defines a new physical layer for 802.11 networks to operate in the 60 GHz spectrum. This frequency band has significantly different propagation characteristics than the 2.4 GHz and 5 GHz bands where wi-fi networks operate. Products adopting the 802.11ad standard will be branded as ” WiGig” capable devices. These will communicate in high speeds targeting short range communications. Theoretically, this standard will be able to transmit data rate of 7 Gbps.
802.11ah: 802.11ah is the first Wi-Fi specification to operate in frequency bands below one gigahertz (900 MHz), and it has a range of nearly twice that of other Wi-Fi technologies. It’s also able to penetrate walls and other barriers considerably better than previous Wi-Fi standards. This is also called as wifi-HALO.
802.11 ax : 802.11ax is the successor to 802.11ac, and will increase the efficiency of WLAN networks. This standard is currently at a very early stage of development by IEEE and this standard aims of providing 4x the throughput of 802.11ac.
802.11ay : 802.11ay is a standard currently under development. It is an amendment that defines a new physical layer for 802.11 networks to operate in the 60 GHz spectrum. This will be an extension of the existing 11ad standards, this is aimed to extend the throughput, range and use-cases. This standard can find its usage in indoor operation, outdoor back-haul and short range communications.
802.11k: 802.11k is an amendment to IEEE-802.11-2007 standard for radio resource management. It defines and exposes radio and network information to facilitate the management and maintenance of a mobile Wireless LAN. 802.11k was incorporated in IEEE Std 802.11-2012. 802.11k is an industry standards that enable seamless Basic Service Set (BSS) transitions in the WLAN environment. The 802.11k standard provides information to discover the best available access point.
802.11r: 802.11r also termed as fast transition (FT) is an amendment to the IEEE 802.11 standard in 2008 , this feature aims to permit continuous connectivity aboard wireless devices in motion, with fast and secure handoffs from one base station to another managed in a seamless manner.
802.11v: 802.11v is a Wireless standard for Network Management under IEEE 802.11 family. This standard allows client devices to exchange information about the network including information about the RF environment, empowering each client to be network aware, which in turn facilitates the overall improvement of the wireless network.
What can we expect in future from wifi technology is defined well in this image below: