Wi-Fi is a trademark for a family of wireless networking standards, all formally defined under the IEEE 802.11 specification. Since the original 802.11 standard was first published, successive generations have each been given their own letter suffix appended to that base number — 802.11b, 802.11g, 802.11n, 802.11ac, 802.11ax and others — and that letter isn't a cosmetic version label; it identifies a genuinely distinct technical specification, with real, measurable differences in maximum data speed, effective range and how many devices the standard can support well at once.
Each letter marks a real technical upgrade, not just a marketing refresh
Each successive lettered 802.11 standard was developed to improve on real, specific limitations of the ones before it, generally through some combination of using additional radio frequency bands, more efficient methods of encoding data into a radio signal, and improved techniques for handling multiple simultaneous device connections without one device's traffic significantly degrading another's. Because each letter corresponds to a genuinely distinct underlying technical specification, a device only supporting an older standard, like 802.11n, simply can't take advantage of a newer standard's real improvements even if it's connected to a router that does support the newer one — the connection between two devices is generally limited to whichever standard both the router and the specific connecting device actually have in common.
Newer letters generally trade some range for considerably more throughput
A recurring pattern across successive 802.11 generations is a general shift toward using higher radio frequency bands to achieve significantly greater data throughput, since higher frequency bands can carry substantially more data per second, but at some cost to a signal's effective range and its ability to penetrate solid obstacles like walls compared with the lower frequency bands used by earlier standards. This is a large part of why real-world Wi-Fi performance can vary so significantly depending on a device's exact distance and the number of walls between it and the router, even when using the very latest available standard — the same newer standard capable of delivering dramatically higher speeds at close range, under ideal signal conditions, doesn't automatically deliver a correspondingly large range improvement to go along with it.
What we're still unsure about
The 802.11 lettering system, and the general pattern of successive generations improving speed and capacity often at some cost to range, are well-documented, standardised networking engineering, published by the IEEE. What's more genuinely uncertain, and depends heavily on the specific real environment a network is deployed in, is exactly how much of a newer standard's theoretical maximum performance improvement a given real household or office will actually realise in day-to-day practice — factors like a building's specific construction materials, how many other nearby wireless networks are competing for the same limited radio spectrum, and how many devices are actually connected at once all shape real-world performance considerably, in ways that a standard's published theoretical specifications alone don't fully predict for any one specific location.
This sits inside Wireless Networking & Wi-Fi Standards, one of seven topics in Networking, one of seven domains in Computer Science, one of seventeen subjects the app can quiz you on.