There are books
on that subject ... and many of them are on-line. Start by searching for LDAP
, look at the OpenLDAP web-site
, and follow the various "References" links. Also check out the various Wikipedias, here and elsewhere.
The essential idea is that authentication occurs through a central server (or distributed network). PAM
(Pluggable Authentication Modules) is the customary Linux mechanism
for interfacing to it, since PAM can handle authentication generically for any application that's aware of PAM. (The applications "just ask PAM," and PAM asks LDAP or whatever.)