Nintendo Network ID

Nintendo Network IDs (or NNID for short) are used to make accounts on Wii U or 3DS systems. These are needed in order to use Nintendo's online service' such as playing games online, buying games on the Nintendo eShop, making friends on the friends list, etc. Your NNID also doubles as a Miiverse account. If a user already has a NNID, the user can link the account so that they can use it on both their Wii U and 3DS. You can also use your NNID to log into your "Nintendo Account".

Miiverse
In order to access Miiverse, users must create an NNID and a Mii first (from the Mii Maker app on both Wii U and 3DS). After the user has created an NNID, they can access Miiverse by clicking the MIiverse icon in the Wii U or 3DS menu.

Miiverse users often create extra NNIDs to access Miiverse in case they get banned. This is against the Miiverse Code of Conduct, but users used to do it regardless because to the immense growth of false reporters.

However, since June 24, 2015, if a Nintendo Network ID gets banned from Miiverse, all of the other NNIDs linked to the device get banned as well (see here for more info).

There was an update released for Miiverse that when your about to post something, it says: Please note that exchanging Nintendo Network IDs and friend codes is prohibited by the Miiverse Code of Conduct.

Blocked NNID
There have been times when a user gets their NNID blocked, it's usually because the user's NNID had an inappropriate name or because they said they were 13+, but then revealed their real age. When this happens, users aren't able to access their NNIDS on their Wii U or 3DS, forcing them to either call Nintendo or create a new NNID. The user with a blocked NNID receives an error message when they try to access any features that require a NNID. Blocked NNIDs are NNIDs that are in the process of being deleted, but haven't yet, and are restricted from being used. After a few months, an email is sent to the owner of the NNID saying that the deletion of the NNID was successful.

Trivia:

 * When your NNID is blocked, you lose your money and games on that Nintendo Network ID, and cannot access the eShop.
 * You lose all registered friends and cannot access your friend list when your NNID is blocked and deleted.
 * The abbreviation for Nintendo Network ID is NNID.
 * You cannot use online features in games when your NNID is blocked and deleted.
 * You cannot access Miiverse when your NNID is blocked and deleted.
 * You're not allowed to share your NNID in posts, even though it's already out in the open.