ISPMan is a distributed system to manage components of ISP from a central management interface.
Front end servers are the machines that are directly visible to your internet users. For example your web (eg. Apache), SMTP (eg. Postfix), DNS (eg. Bind) servers while backend servers can be Mailbox servers (eg. Cyrus IMAP server), Fileservers, database servers etc.
ISPMan is designed to be scalable.
ISPMan is writtin mostly in Perl and is based on four major components. All these components are based on open standards and are easily customizable.
There is some further information in this article: http://www.linuxfocus.org/Deutsch/September2000/article173.shtml