The software seeks to combine a full featureset with a lightweight code base while being easy to learn, use and extend.

The source code (~83KB) is written almost entirely in PHP and engineered specifically with user experience and usability as its primary design goals.

The lightweight code base[7] is controlled and maintained by a small core team, with additional functionality and funding contributions coming from various enterprises and user groups,[8] who also help guide its future direction.

Fat-Free also has a number of plug-ins that extend its functionality as well as data mappers for SQL and NoSQL database back-ends: SQLite, MySQL, PostgreSQL, MSSQL, Sybase, DB2, MongoDB, CouchDB, and Flat File.

The core functionality is accompanied by a number of optional plug-ins, among them a template engine, a Unit testing toolkit, Database-managed sessions, Markdown-to-HTML converter, Atom/RSS feed reader, Image processor, Geodata handler, a Basket/Shopping cart application and data validation.