rmw defines an interface of middleware primitives that are used by the higher level ROS API's. It consists of these main components:

It also has some machinery that is necessary to wait on and act on these concepts:

Initialization and shutdown management (global for now)
- rmw/rmw.h
Wait sets for waiting on messages and service requests / responses to be ready
- rmw/rmw.h
Guard conditions for waking up wait sets asynchronously
- rmw/rmw.h

Further still there are some useful abstractions and utilities:

Allocator functions for various types
- rmw/allocators.h
Error handling functionality (C style)
- rmw/error_handling.h
Macros
- rmw/macros.h
Return code types and other utility types
- rmw/types.h
Sanity checks for some of the types
- rmw/sanity_checks.h
Macros for controlling symbol visibility on the library
- rmw/visibility_control.h
Utility function to demangle a type to a human readable string (C++ specific):
- rmw/impl/cpp/demangle.hpp