Wi-Fi standards decide the speed and range of a Wi-Fi network. Generally later standards are backward compatible with earlier ones.
802.11b: This was the first commercialized wireless standard. It offers a top speed of 11 Mbps and operates only on the 2.4 GHz frequency band. The standard was first available in 1999 and is now totally obsolete; 802.11b clients, however, are still supported by access points of later Wi-Fi standards.
802.11a: Similar to 802.11b in terms of age, 802.11a offers a speed cap of 54 Mbps at the expense of much shorter range, and uses the 5 GHz band. It's also now obsolete, though it's still supported by new access points for backward compatibility.
802.11g: Introduced in 2003, the 802.11g standard marked the first time wireless networking was called Wi-Fi. The standard offers the top speed of 54 Mbps but operates on the 2.4 GHz band, hence permitting better range than the 802.11a standard. It's used by many older mobile devices, such as the iPhone 3G and the iPhone 3Gs. This standard is supported by access points of later standards. 802.11g is also becoming obsolete.
802.11n or Wireless-N: Available since 2009, 802.11n has been the most popular Wi-Fi standard, with lots of improvements over the previous ones, such as making the range of the 5 GHz band more comparable to that of the 2.4 GHz band. The standard operates on both 2.4 GHz and 5 GHz bands and started a new era of dual-band routers, which accommodate two access points, one for each band. There are two types of dual-band routers: selectable dual-band routers (now defunct) that can operate in one band at a time and true dual-band routers that simultaneously transmit Wi-Fi signals on both bands.
On each band, the Wireless-N standard is available in three setups, depending on the number of spatial streams being used: single-stream (1x1), dual-stream (2x2) and three-stream (3x3), offering cap speeds of 150 Mbps, 300 Mbps and 450 Mbps, respectively. This in turns creates three types of true dual-band routers: N600 (each of the two bands offers a 300 Mbps speed cap), N750 (one band has a 300 Mbps speed cap while the other caps at 450 Mbps) and N900 (each of the two bands allows up to 450 Mbps cap speed).
Note: In order to create a Wi-Fi connection, both the access point (router) and the client need to operate on the same frequency band. For example, a 2.4 GHz client, such as an iPhone 4, won't be able to connect to a 5 GHz access point. Also, a Wi-Fi connection takes place on just one band at a time. If you have a dual-band capable client (such as the iPhone 6) with a dual-band router, the two will connect on just one band, likely the 5 Ghz.
802.11ac: Sometimes referred to as 5G Wi-Fi, this latest Wi-Fi standard operates only on the 5 GHz frequency band and currently offers Wi-Fi speeds of up to 2,167 Mbps (or even faster with latest chip) when used in the quad-stream (4x4) setup. The standard also comes with the 3x3, 2x2, 1x1 setups that cap at 1,300 Mbps, 900 Mbps and 450 Mbps, respectively.
Technically, each spatial stream of the 802.11ac standard is about four times faster than that of the 802.11n (or Wireless-N) standard, and therefore is much better for battery life (since it has to work less to deliver the same amount of data). In real-world testing so far, with the same amount of streams, I've found that 802.11ac is about three times the speed of Wireless-N, which is still very good. (Note that the real-world sustained speeds of wireless standards are always much lower than the theoretical speed cap. This is partly because the cap speed is determined in controlled, interference-free environments.) The fastest peak real-world speed of an 802.11ac connection I've seen so far is around 90 MBps (or 720 Mbps), which is close to that of a Gigabit Ethernet wired connection.
On the same 5 GHz band, 802.11ac devices are backward-compatible with Wireless-N and 802.11a devices. While 802.11ac is not available on the 2.4 GHz band, for compatibility purposes, an 802.11ac router can also serve as a Wireless-N access point. That said, all 802.11ac chips on the market support both 802.11ac and 802.11n Wi-Fi standards.
802.11ad or WiGig: First introduced in 2009, the 802.11ad wireless networking standard became part of the Wi-Fi ecosystem at CES 2013. Prior to that, it was considered a different type of wireless networking. 2016 marked the year when the first 802.11ad router, the TP-Link Talon AD7200, became available.
Operating in the 60 Ghz frequency band, the 802.11ad Wi-Fi standard has an extremely high speed -- up to 7 Gbps -- but a disappointingly short range (about one-tenth of 802.11ac.) It can't penetrate walls very well, either. For this reason, the new standard is a supplement to the existing 802.11ac standard and is intended for devices that sit within a close proximity of the router.
It's an ideal wireless solution for devices at a close range, with a clear line of sight (no obstacles in between) such as between a laptop and its base-station, or a set-top box and a big screen TV. All 802.11ad routers will also work as 802.11ac routers and support all existing Wi-Fi clients, but only 802.11ad devices can connect to the router at high speed over the 60 Ghz band.
802.11ax: This is the next generation of Wi-Fi, set to supersede 802.11ac. Like 802.11ac, the new 802.11ax is backward compatible with previous Wi-Fi generations. However, it's the first standard that focuses not only on faster speed but also on Wi-Fi efficiency, especially in crowded air space. In other words, 802.11ax aims to maintain network capacity even during less than ideal conditions. Ultimately, this means it allows for higher ratio of real-world speed versus theoretical ceiling speed. It's also said to reduce energy consumption by two thirds compared to 802.11ac, which is great news for mobile users.
On paper, 802.11ax can be four times faster than
802.11ac, up to some 5 Gbps. Also, an 802.11ax router can boost existing
pre-802.11ax Wi-Fi devices' real-world speeds thanks to its ability to manage
traffic diversity in dense, overlapping networks. 2017 is the year that
networking chip makers, such as Qualcomm, introduced their first 802.11ax
chips. That said, consumer devices that support 802.11ax are predicted to be
available by the end of 2017 or early 2018.