This directory contains Chrome Foundation Services. If you think of Chrome as a “portable OS,” Chrome Foundation Services can be thought of as that OS' foundational “system services” layer.
Roughly each subdirectory here corresponds to a service that:
//services/shell with its own unique Identity.Individual services are structured like so:
//services/foo/ <-- Implementation code, may have subdirs.
/public/
/cpp/ <-- C++ client libraries (optional)
/interfaces/ <-- Mojom interfaces
Code within //services may only depend on each other via each other's /public/ directories, i.e. implementation code cannot be shared directly.
Service code should also take care to tightly limit the dependencies on static libraries from outside of //services. Dependencies to large platform layers like //content, //chrome or //third_party/WebKit must be avoided.
Note that while it may be possible to build a discrete physical package (DSO) for each service, products consuming these services may package them differently, e.g. by combining them into a single package.
Services can be thought of as integrators of library code from across the Chromium repository, most commonly //base and //mojo (obviously) but for each service also //components, //ui, etc in accordance with the functionality they provide.
Not everything in //components is automatically a service in its own right. Think of //components as sort of like a //lib. Individual //components can define, implement and use mojom interfaces, but only //services have unique identities with the Mojo Shell.