A server side open source CMS (Content Management System). I am not familiar with it,
but I would guess it keeps content and markup more separate than with traditional
CSS (Cascading Style Sheets) schemes. Joomla itself in written in
PHP (Pre-Hypertext Processor). It is reputedly easier to maintain and eaiser to use
that traditonal HTML (Hypertext Markup Language) and it
generates websites that are not easily hacked.