Seen from a more pragmatic point of view, it's a set of loosely bound macros and functions which's actual use depends on how creative one's able to combine them. They can, however, serve as a very competent base for quick coding of mandatory engine features. For example, most of Imperial Graphics Engine's functionality (which can also be found here) was coded in just two days using ITK. More features will be added in the future if I can find worthwhile, reusable additions. There's a basic doxygen-style documentation within the code, but it doesn't cover all details yet as well as important things like certain function call orders. ITK's also not free of bugs and it shoud be used with care in the current release.
Currently covered functionalities:
Planned and WIP modules
Modified BSD License
ITK Release 0b