What's the difference between 'passwd' and 'chpasswd'?

From man chpasswd:

'This command is intended to be used in a large system environment where many accounts are created at a single time.'

passwd is (in my experience) normally used interactively for a single user.

See man pages for both of them:

NAME

chpasswd - update passwords in batch mode

SYNOPSIS

chpasswd [options]

DESCRIPTION

The chpasswd command reads a list of user name and password pairs from standard input and uses this information to update a group of existing users.  Each line is of the format:

   user_name:password

---

NAME

passwd - change user password

SYNOPSIS

passwd [options] [LOGIN]

DESCRIPTION

The passwd command changes passwords for user accounts.  A normal user may only change the password for his/her own account, while the superuser may change the password for any account.  passwd also changes the account or associated password validity period.

In a nutshell:

• passwd checks if the STDIN (file descriptor 0) is attached to the terminal, using isatty(0). If not, passwd would bail out i.e. you can only work with passwd interactively

• chpasswd, on the other hand, is designed to read (username and) password from STDIN, and is called the batch mode (creates/updates multiple user credentials at once). It reads password(s) (in clear text by default), and username(s) from STDIN, given in the format user_name:password, with newline separating the entries