Joomla! is a content management system or operating system.  The easiest way to think of it is you need Windows installed on a PC to make it do anything, well you need an operating system installed on the Internet to be able to do anything.  The reasons we use Joomla! are that it is free and that it is widely supported.  The code Joomla! is made with is what is called 'Open Source' this means that like minded individual developers get together accross the world via the Internet to create Joomla! and its associated extensions.  Try Googling Joomla! or take a visit to Amazon and have a look at all the books written on the subject.  There are many content management systems out there some good, some bad, some free, some not so free but with out question Joomla! is the best supported and most versitile.