CMS stands for Content Management System. A CMS is software for managing content on a website directly on the server through a browser interface. It typically stores the material in a database, generating pages for users on request. Joomla! is an open source Content Management System (CMS) designed to assist users in building Web sites and other online applications. Joomla is a continuation of the work of the Development Team that unanimously resigned from the Mambo project in August 2005. Joomla version 1.0 is derived from Mambo 4.5.2.3 but includes many additional bug fixes and security patches. The name Joomla is a phonetic spelling for the Swahili word "Jumla", which means "all together" or "as a whole".
CMS is content Management System - and Joomla is a type of CMS. Usually it is a combination of PHP and SQL which allows you to easily manage a large website. There are templates for various pages and built in links etc. If you have ever used Wordpress on your own website then you have an idea of the sort of thing. That is the brief and simple version - I can get a lot more complex!
i have some sites maded with joomla if you want to chek out Joomla's posibilities: - http://www.riveresmundial.com.ar/ - http://www.xiondesigns.com.ar/ - http://www.solovuelosbaratos.com.ar/ if you need any help just ask me
For a social networking site more suitable http://www.pligg.com/ or drupal.org both are very powerful and flexible.