3.5mm jacks or sockets are the most common audio ports on current electronic devices, due to their compatibility with most wired headphones. Also known as mini jack sockets and headphone jacks, 3.5mm jacks originate from 19th-century phone switchboards. When connected to a compatible plug, headphone jacks work by allowing an electric current to run between the linked devices and generate sound. Connected devices may be, for instance, a smartphone and a pair of headphones, or even a microphone.
Plugs that connect to mini jack sockets are not always the same. Quite rare these days, 3.5mm mono plugs (TS) have one ring close to its tip, whereas the popular 3.5mm stereo plugs (TRS) have two rings. Some products, such as Apple's wired earphones, have an extra ring. Furthermore, the number of wires running inside each plug and cable is not always the same.
A headphone jack used to be a given, and it was hard to imagine that they would ever be gone. We are, however, slowly approaching such a time, at least when it comes to smartphones. Thanks to the evolution of wireless technology and headphones, as well as to the industry's necessity to produce slimmer and impermeable devices, a growing number of smartphones are being released without 3.5mm jacks.
So far, the annihilation of 3.5mm jacks hasn't had an impact on the tablets market. Headphone jacks are still available in most models, although, as it's always the case with the tech industry, this could change in the future.