A marking gauge has a sharp pin as opposed to a cutter.  It is suitable for marking out parallel lines with the grain or across end grain say in marking out a tenon but it is not suitable for marking out lines across the grain to form a shoulder as it leaves a ragged line.

 It is also very useful for accurately marking out a single point or series of single points parallel to an edge to position screws etc.

Refer to "USING A CUTTING GAUGE"  for further guidance on using this tool.