kBuild is a build framework based on a modified GNU make and a standard set of cross platform tools. VirtualBox is a cross-platform project and therefore we needed a powerful build system that works on several platforms.
kBuild is extremely powerful and tries to hide most of its complexity in templates so that the actual makefiles are relatively easy to understand and write. The core of kBuild is very complex though and there are currently 0.5 people on the planet that fully understand it.
In order to make life easier, the kBuild repository contains precompiled binaries of all tools for all supported platforms. In case you prefer to build those yourself, you may download the complete sources of kBuild from its external project site.
kBuild is also used outside VirtualBox and maintained as a hobby by Knut St. Osmundsen. The kBuild project page can be found at http://svn.netlabs.org/kbuild.