linux user manager

there are some users in linux ,so we need to manage them well.I  give some advice in the following
first ,input commands
more  /etc/passwd
note:when a user login in with name and passoword.linux will find this exits in the passwd file or not.if exits ,it will read usrID and groupID. in the same time ,the root user and shell  also   appear.
next linux will check the passwd in the /etc/shadow  ,if it is correct ,then we will login the shell.