#define variable 1

#ifdef variable      //if variable is defined as 1?
//code
#endif

#ifdef _WIN32
/* Something specific to windows. */
#else
/* Something not available on windows. */
#endif

#ifdef ...

if (...)

#if SOMETHING
    do_some_c_stuff();
#endif

#ifdef SOMETHING
    do_some_c_stuff();
#endif

#ifdef SOMETHING
#if SOMETHING
    do_some_c_stuff();
#endif
#endif

#if DEBUG
    // Do some additional logging here we don't want to to in release.
#endif