What is htpasswd ?
htpasswd is a command tool for the open source http server apache httpd to generate password files for basic http authentication.
What is the difference between the encryption methods ?
MD5: Passwords are encrypted using MD5. On Windows, Netware and TPF, this is the default encryption method.
crypt: Use crypt() to encrypt the password. This is the default on platforms other than Windows, Netware and TPF. Although it is supported by htpasswd on all platforms, it is not supported by httpd servers on Windows, Netware and TPF.
SHA: Use SHA encrypted passwords. It is designed to facilitate migration or porting to Netscape using the LDAP Directory Interchange Format (ldif).
plain: Not encrypted, use plain text passwords. Although htpasswd can create such passwords on all platforms, the httpd backend only supports plain text passwords on Windows, Netware and TPF.
Why make it online?
If we don't use apache server, for example, using nginx, etc., we may not have this command line tool at hand, so we can't generate the password file, and having the online version can be convenient for server administrators to use.