WordPress is a popular Content Management System that runs on top of LAMP Technology. The basics beyond WordPress is they created a simple to use sytem of content management that is focussed on power weblogs, or online journals.
The front end is powered by Themes, that are called WordPress Themes and are a popular industry led by Premium (not free) Themes such as the Thesis Theme by DIYThemes.
There is a lot of controversy about whether or not WordPress Themes have to be GPL, since they are built on top of the WordPress CMS which is GPL.
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