User accounts on the Switch are separated into three separate IDs: a local user, a Nintendo Account, and an online User ID.
Local Users on the Switch are the primary user accounts on the Switch. These accounts are created on device setup, during which you can set a name, profile icon (must pick from a predetermined set or create a custom Mii), and region. Multiple local users can be created, each with their own separate name and icon. Local user accounts are issued a Friend Code, which can be used to add Friends to system, similar to the Nintendo 3DS. The maximum number of local users you can have is TBD. You can have multiple users with different regions; to do so, change the system region (and presumably, system language for some regions), create a new user, and have a Nintendo Account from that region to login to. After the user is created and you've logged into the eShop with your Nintendo Account in the target region, changing the region and system language should no longer be necessary to access either region's eShop.
Friend Codes are a unique twelve character ID, prepended by “SW-”, issued to each user account. The format is SW-1234-1234-1234. How these Friend Codes are generated is currently unknown. Friend Codes are currently the primary method to add Friends to the system. By adding someone else's Friend Code to your system, a Friend Request will be sent to them that they can accept or reject.
Nintendo Accounts are general accounts for all Nintendo services; they are used for Nintendo's smartphone applications, can be linked to Nintendo Network IDs, and are required to be able to create an online User ID for the Switch, as well as access online services like the Nintendo eShop. Nintendo Accounts are located here. To create an account, you must enter a birthday, email, gender, nickname, password, region, and time zone.
Though these were used for online services on the 3DS and Wii U, they have been replaced by User IDs on the Switch and will presumably be deprecated when those consoles hit end-of-life. If you have an NNID linked to your Nintendo Account, you can merge any funds it has available in its Wallet with your main Nintendo Account Wallet, allowing those funds to be used on the 3DS, Switch, and Wii U.
User IDs are a sub-ID under your Nintendo Account and are currently used solely for the Switch. Presumably, these are the replacement for Nintendo Network IDs and will be used as the primary method of online user identification on the Switch. User IDs were added to the User Info page for Nintendo Accounts at some point on or before February 20, 20171). If you've created a User ID, by adding a Nintendo Account to your Local User on the Switch, its User ID will be linked as well.
When creating a User ID, a minimum length of six characters and a maximum length of thirty characters is specified. However, the validation for minimum length is done on the client-side; if you edit the HTML of the page to change the minimum length and re-enable the submit button, you can create a five character User ID. The server will not accept one to four character IDs regardless, however; attempting to create them leads to a redirect page that says “Please enter six characters.” or “Access denied.”. These shorter-than-intended five character User IDs do not seem to cause any problems, they can be used on the Switch without issue. Though, the censored ID shown in User Settings will be shown as if it has six characters, i.e. a User ID called “user1” will become “use···”.