Nintendo's new service costs $20 per year ($4/month, $8/three months), and is planned for launch in 2018. When it arrives, it'll only be available on the Nintendo Switch — Nintendo's newest game console, which operates as a portable handheld and a home console.
Beyond access to online gaming — which is free for now on the Switch — you'll also get access to a library of classic games from Nintendo's history. The first three games Nintendo showed off were "Super Mario Bros. 3," "Dr. Mario," and "Balloon Fight," but more are expected to be announced as we learn more about the service.
Notably, those classic games are all from the NES — Nintendo's oldest home console. Nintendo has yet to confirm if games from the SNES, N64, GameCube, Wii, or Wii U will appear on the service (to say nothing of Nintendo's handheld gaming library appearing). There is no release date for the service outside of "2018" thus far.