SMF forum
Definition
Simple Machines Forum is a free professional software package that allows users to create their own community or Forum on-line in few minutes.
SMF features
Very powerful system and personalization of themes enables the users to control fully (total control of the forum) the message's lay-out.
SSI (function)
Server Side Includes is the function that let user's forums and website to interact to each other.
SMF Programming Language
SMF is written in the very popular programming language PHP and use MySQL database and it is projected to give the users all the functionalities that a user needs.
References
http://en.wikipedia.org/wiki/Simple_Machines_Forums
http://www.simplemachines.org/community/index.php