Simple definition
Linux Name Service Switch controls how applications resolve user, group, host, and other identity data.
Purpose and use
NSS, Name Service Switch, defines how system libraries look up information such as users, groups and hostnames.
How does it work?
Configuration specifies sources and ordering, for example local files, DNS or a directory service.
Example and verification
A directory identity missing from system lookups may indicate an NSS-source problem.
Points to watch
Resolving a user does not prove that authentication will succeed or that an application permits sign-in.
In short
NSS organizes information lookup; authentication and authorization remain separate.