Here is a list of all namespaces with brief descriptions:

[detail level 123]

irr	Everything in the Irrlicht Engine can be found in this namespace
core	Basic classes such as vectors, planes, arrays, lists, and so on can be found in this namespace
gui	The gui namespace contains useful classes for easy creation of a graphical user interface
io	This namespace provides interfaces for input/output: Reading and writing files, accessing zip archives, xml files, ..
scene	All scene management can be found in this namespace: Mesh loading, special scene nodes like octrees and billboards, ..
quake3
video	The video namespace contains classes for accessing the video driver. All 2d and 3d rendering is done here