WiFi, otherwise known at the 802.11 protocol, has had 18 versions. This page covers just four and compares the frequencies, theoretical bitrate, and security aspects.
The versions we are covering are 802.11g (2003), 802.11n (2009), 802.11ac (2014) and 802.11ax (2019).
When discussing frequencies it should be noted within each frequency multiple channels are used. For example for 2.4GHz, the starting frequency is 2.401GHz and the end is 2.495GHz – between these frequencies 14 channels are available, typically, however, only 3 blocks of channels are used to separate the traffic and reduce the interference between the channels. 5GHz has 23+ channels.
The benefit of the higher 5GHz frequency is it typically has a greater range (distance) than 2.4GHz, however, 5GHz is more easily absorbed by obstacles than 2.4GHz. 2.4GHz is more likely to be interferred with by microwaves and cordless phones (not mobile phones).
The original method of securing a WiFi connection was using WEP (Wired Equivalent Privacy), this is easily hacked and should not be used – even though it is often available. The current WPA2 (WiFi Protected Access) is the current best practice (even though a hack was discovered in 2017).
All bitrates mentioned on this page are theoretical and rarely achieved in the real world due to interference and other factors.
802.11g uses 2.4GHz frequency has a maximum theoretical bitrate of 54 Mega bits per second (Mbps). It is rarely used now.
802.11n uses both 2.4GHz and 5GHz and due to a wider range of frequencies can achieve a theoretical bitrate of 600Mbps. This version introduced MIMO (Multi-In Multi-Out) which meant multiple antennas could handle multiple connections, allowing for a greater number of devices to connect simultaneously. Previously only one device could talk to the Access Point at one time, now multiple devices can.
802.11ac uses the 5GHz frequency and has a theoretical maximum bitrate of 6.9Gbps. This is currently the most commonly used version.
802.11ax uses 2.4GHz, 5GHz plus 1 and 6 GHz. The 2.4GHz and 5GHz are there so it is backwards compatible. It has a theoretical maximum bitrate of 9.6Gbps. As this is so new not many devices (as of 2020) use this version.