Despite the fact it's been nearly a year since it launched, the Nintendo Switch still doesn't have a dedicated online service ala Xbox Live or PS Plus. Nintendo initially promised it would arrive at an unspecified point in early 2018, but now it's confirmed that date has been pushed back to September.
— Nintendo UK (@NintendoUK) February 1, 2018
In the past Nintendo announced that it would cost $20 a year, which probably means we'll be paying anything from £15-£20 here in the UK. The main focus of that subscription is that it lets people play Switch games online, since free access will expire once the premium service actually launches.
The biggest perk, however, is that Switch Online will give subscribers access to classic Nintendo games with new online features. We don't know the full extent of that just yet, but in the past Nintendo has mentioned Super Mario Bros. 3, Balloon Fight and Dr. Mario would be included. Players will have unlimited access to all the available games, provided of course that they maintain an active subscription.
What definitely won't be included are things like voice chat and lobby features, which Nintendo has chosen to route through paired smartphone apps. The dedicated Switch Online app was released alongside Splatoon 2 last July on Android and iOS.