Thread: what does the ":" do in this code...

what does the ":" do in this code...

Code:

	union aword {
	  DWORD dword;
	  BYTE byte [4];
	  struct {
	    unsigned int byte3:8;
	    unsigned int byte2:8;
	    unsigned int byte1:8;
	    unsigned int byte0:8;
	  } w;
	};