A Flexible Content Element is a content element that is defined and programmed by a TYPO3 developer using the TemplaVoila extension. Like any of the default content elements, an FCE can be used within the content section of a page.
With this very flexible templating tool, a TYPO3 editor can, for example, arrange a text in two columns very easily: all she needs is an FCE that consists of two
div containers ('floating' and 50% width each). These two containers (e.g. named 'Column left' and 'Column right') need to be defined in TemplaVoila before. Their content type should be 'Content Elements', which allows an editor to fill them with content elements of any kind. Once this FCE is inserted into a TYPO3 page, the two sections 'Column left' and 'Column right' appear and can be filled with various content elements — even this or other FCEs.
That was only a simple example of the almost limitless range of application of Flexible Content Elements. Just looking at the possibility of content nesting, it is obvious that layouts can be made which, using the default way of TYPO3 templating, are very complex or impossible to achieve.