Drake's C++ library is composed primarily of an interface for solving numerical optimization problems, interfaces for modeling dynamical systems, and a collection of state-of-the-art algorithms for optimization on dynamical systems:
For more general information, you can also visit the Drake documentation main page.
If you want to make your own Doxygen locally, see Documentation Generation Instructions
Check out the Doxygen C++ documentation
Drake's Doxygen documentation is hosted online for the master branch, but is only updated nightly.