Many Architecture Definition Languages have been created that can be used to formally define the architecture of a system. For some, however, getting started in a good way while strictly following a modeling language is a bigger challenge than expected for those with little or no experience with modeling. There are many reasons for this, ranging from typical real-world time and budget pressures to a lack of perceived benefit from creating a formal description of a system that isn’t necessarily reflective of, or connected to, the source code.

Known notations like UML, SysML or ArchiMate present a similar challenge. They are powerful and fulfill all relevant aspects of Software, Systems or IT Infrastructure, but teams often don't know these notations well enough and, as a result, perceive them to be too complicated or incompatible with agile approaches…and they may not have the required tooling.

The C4 model
The C4 model is a lightweight approach created by Simon Brown to help software development teams describe and communicate software architecture, both during up-front design sessions and when retrospectively documenting an existing code-base. It's a way to create maps of your code, at various levels of detail, in the same way you would use something like Google Maps to zoom in and out of an area you are interested in.

In order to create these maps of your code, we first need a common set of abstractions to create a language that we can use to describe the static structure of a software system.

C4 diagrams
The C4 model considers the static structures of a software system in terms of containers, components and classes (or code). Visualizing this hierarchy of abstractions is then done by creating a collection of core diagrams namely Context, Container, Component and optionally Class. This is where the C4 model gets its name from. Furthermore, you can supplement the core C4 diagrams with other diagrams to show other aspects as well.