Jig will only hold the workpiece and guide the tool. It will not hold the Tool.
Jigs and fixture increases accuracy. Jigs are holding device and fixture are generally used for bigger work piece. Jigs are preferred over fixture in drilling because they also help in guidance the tool.
Jigs are not used in Milling machine.
Fixture is a work-holding device that only holds, supports, and locates the workpiece and does not guide the tool.
A jig is a work-holding device that holds, supports, and locates the workpiece and guides the one or more tools to perform a specific operation.