The kernel and the userspace applications are written from scratch in C++ and Assembly (and some C). The kernel is not based on any existing kernel and not Unix-compatible. There is a relatively small POSIX.1 compatibility layer that allows porting some software to the system though.
The most advanced features are SMP support (symmetric multiprocessing) to run on multiple processors, a v8086 monitor for executing BIOS calls (which also introduced VESA support), support for ELF binary loading (including thread-local-storage and all the other little things), as well as signal support. And there is a fancy little GUI that makes it all good-looking :-)