passwd contains 7 fields:
username - unique login name
password - empty, moved to /etc/shadow
user id - unique number id
group id - id of primary group
GECOS - long name
home directory - login directory
login shell - login interpreter
shadow contains the actual passwords in hashed form. Each line contains 8 fields:
username - unique login name
password - hashed password value
lastchanged - days since password change
minimum - minimum number of days between password changes
maximum - maximum number of days between password changes
warn - number of days before password expiration to warn user
inactive - number of days since expiration
expire - absolute expiration date
group file contains group membership for all local users and it has 4 fields:
group - group name
password - empty
group id (GID) - unique group id number
group list - a comma delimited list of usernames that belong to the group