Gnu Compiler Collection
The 
GNU Compiler Collection is a toolkit for managing compilation and assembly, and is one of the core utilities of the Free Software Foundation's GNU Project, along with the GNU EMACS editor, the BourneAgainShell (bash), and the binutils toolkit.
While it is most closely associated with C and C++, it is in fact a suite of several compilers and their libraries; the base package for version 3.3.2 includes support for C, C++, Objective C, Java, Fortran 77, and Ada, and several other compilers can be used with gcc as well.
While gcc was developed for Unix and it's derivatives, it is highly portable and can usually be ported to any system that supports a shell interface. It's code generation is also retargettable, and can support most common hardware platforms.
OSFAQ and GCC
Although other compilers can be used for OS development, we encourage you to use one of GCC derivatives. It comes installed by default on virtually every Unix system and good ports exist for Windows too.
- DJGPP is old and obsolete under windows. Don't use it.
- MinGW does a good job for win32 applications, not for osdev.
- Cygwin is the choice for OS dev'ing under windows
Finally, setting up a cross-compiler environment is recommended in order to save you further headaches (Note that on Windows, a cross compiler can only be built in the Cygwin environment).