Concrete 5 is a long running content management system. It is known for it's user friendly interface and with it's recent iterations has moved to adopt Symfony components.
Concrete 5 is a Content Management System in development since 2003. It focuses on providing an easy to use experience for content editors. The tool is designed to allow users to assemble pages from individual blocks without developer involvement.
The community has a market place for ready made themes and extensions (Add ons), which can be used to extend the system. The underlying architecture also allows developers to customize the system in a structured way, while maintaining the usability promises of the core tool.
In the later versions Concrete 5 has started adopting components from Symfony and other PHP projects using composer. This has caused some confusion in versioning and has broken backwards compatibility for later versions. However, with core functions like HttpKernel, Class Loading and Routing from Symfony, the tool is now stabilising.