/etc/shadow

The /etc/shadow file is used to store information about user authentication. It requires superuser read permissions.

$ sudo cat /etc/shadow

root:MyEPTEa$6Nonsense:15000:0:99999:7:::

You'll notice that it looks very similar to the contents of /etc/passwd, however in the password field you'll see an encrypted password. The fields are separated by colons as followed:

1. Username
2. Encrypted password
3. Date of last password changed - expressed as the number of days since Jan 1, 1970. If there is a 0 that means the user should change their password the next time they login
4. Minimum password age - Days that a user will have to wait before being able to change their password again
5. Maximum password age - Maximum number of days before a user has to change their password
6. Password warning period - Number of days before a password is going to expire
7. Password inactivity period - Number of days after a password has expired to allow login with their password
8. Account expiration date - date that user will not be able to login
9. Reserved field for future use

In most distributions today, user authentication doesn't rely on just the /etc/shadow file, there are other mechanisms in place such as PAM (Pluggable Authentication Modules) that replace authentication.