The #include command works as in Standard C, with the caveat that the filename must be enclosed in double quotes. There is no notion of a system include path, so enclosing a filename in angle brackets is forbidden.
#include "foo.h" // ok #include <foo.h> // error!
NBC programs can begin with #include "NXTDefs.h" but they don't need to. This standard header file includes many important constants and macros, which form the core NBC API. NBC no longer requires that you manually include the NXTDefs.h header file. Unless you specifically tell the compiler to ignore the standard system files, this header file is included automatically.