Alright, it’s 802.11 but what are the characters? (a, b, g, n, ac)

Originally WiFi or 802.11 was designed for barcode scanners. In a warehouse cords were inpractical, so going cordless was logical. Bandwidth requirements were very modest, so speed was not a primary design objective. This is the basis for all enterprise wireless networks of today providing videoconferencing and high speed database connections. The change has been gradual:

The first redesign was 802.11a. It offered increased speed, but used the new 5 GHz band. 5 GHz radios were expensive and A never really took off. It was B, that used the 2,4 GHz band that really created the market. G brought the speeds of A to the less expensive 2,4 GHz band. N added even more speed and N was defined for both bands. 5 GHz radios were still more expensive, so cheaper devices and access points only had 2,4 GHz radios. The latest is AC, which is only defined for 5 GHz, but in practice all devices and access points are compatible with older standards (so they support also 2,4 GHz).

Isn't the maximum speed of 802.11n 600 Mbps? Yes, it is. N introduced MIMO (Multiple Input, Multiple Output) radios that could use multiple connections (four at max.) 4 x 150 Mbps is 600 Mbps. In AC there can be 8 radios that can connect to multiple clients at the same time (MU-MIMO or Multi-User MIMO). In practice mobile phones have a single radio, tablets may have two and laptops four. Each radio eats up batteries (and also adds to the manufacturing cost).

Why doesn't my device ever report these maximum speeds? In N it was possible to bond two 20 MHz channels to create a 40 MHz channel, which can transfer over twice the data. The maximum speeds are calculated using these wide channels. In AC the maximum is eight channels or 160 MHz channel width. The channel width is set at the access point, so if the access point is only using the default single channel then the speed will be limited to that. The 2,4 GHz band has so few channels that channel bonding is practical only in the 5 GHz band.