These three features are the core features of the family, focused on integrating Fluid with TYPO3. You can find a more detailed description of each individual extension in the main extension overview.
By installing flux you (and you will in this case be a developer) get access to using the included ViewHelpers to define highly advanced, flexible and dynamic forms for content elements (plugins as well as regular content supported). Flux attaches itself to records of a particular type, identified by the table, and hooks into multiple places in the TYPO3 core in order to process Fluid template files and generate TCEforms field configurations.
By using variables defined in the configuration itself, variables coming from a custom provider, variables inserted in TypoScript and even variables inserted by other ViewHelpers (i.e. the as argument on f:for, which adds a template variable which only exists inside the tag) it is even possible to manipulate the form itself - using checkboxes to switch off entire sections or groups of fields, using mode selectors to completely change the available fields, using a "storage page" selector and then using that PID to set a condition for another field - there's almost no limit to how dynamic your forms can be.
By installing the extensions flux and fluidpages you get access to using template files written in Fluid. These template files can contain advanced, dynamic configuration options which are immediately available in the template when it is being rendered - and can even be used to affect the configuration options the page template contains. Together with fluidcontent this feature can effectively replace TemplaVoila with a solution based completely on Fluid.
On TYPO3 6.0 fluidpages even adds the capability to use the flux:flexform.grid ViewHelpers to define a Grid - which will be turned into a Backend Layout (if no Backend Layout record is selected and/or inherited). You guessed it: the Backend Layout defined by such a Grid is also fully dynamic, just like the rest of the configuration options.
This site, and many others along with it, are already powered by fluidpages. In particular this site uses the fluidpages_bootstrap template collection.
By installing flux and fluidcontent you get access to using specially constructed Fluid template files as individual content elements. By using flux an advanced, dynamic set of configuration options become available when editing the content element - and the variables which are configured in each content element become available as template variables when the template is being rendered. Along with fluidpages, fluidcontent can be used to effectively replace TemplaVoila with a solution based completely on Fluid.
Fluid Content Elements can even contain other content elements, much like the Flexible Content Element concept from TemplaVoila. On TYPO3 6.0, native drag and drop support becomes available - on 4.x, using gridelements provides the drag and drop of content elements.
Fluid Content Elements are extensively used on this site - in particular the fluidcontent_bootstrap package is being used for column separators, hero units etc.