Simple definition
SSSD provides cached identity and authentication services for Linux clients using directory providers such as LDAP or Active Directory.
Purpose and use
SSSD, System Security Services Daemon, provides host identity, authentication and authorization services linked to a central directory.
How does it work?
Configured providers supply data, and selected information can be cached for local consumers such as NSS and PAM.
Example and verification
Test account lookup, authentication and access to the relevant service separately.
Points to watch
Caching and offline behavior depend on configuration; cached information does not guarantee current directory authorization.
In short
SSSD connects local applications with central identity while keeping lookup and authorized access distinct.