Use Select and Link and Unlink Selection on the toolbar to make and remove links between objects.
Linking Objects
The general process of creating links is to build the hierarchy from child to parent. You click Select And Link on the toolbar, select one or more objects as children, and then drag the link cursor from the selection to a single parent object. The selected objects become children of the parent object.
Once objects are linked, any transformations applied to the parent are also applied to its children. For example, if you scale the parent to 150%, the size of its children and the distance between the children and the parent are also scaled by 150%.
Unlinking Objects
Click Unlink Selection to remove the link from selected objects to their parents. Any children of the selected object are unaffected.
You can quickly unlink an entire hierarchy by double-clicking the root object to select the object and all of its children. Then click Unlink Selection.
Linking Animated Objects
You should establish links before you begin animating objects. The linkage of objects with Select and Link cannot be animated; the link remains in force throughout the entire animation.
If you want your objects to be linked during one part of the animation but not another, you can a Link constraint to change the linkage at specific frames.
Displaying Links
A complex mesh hierarchy can be displayed with the links visible, or even with the links replacing the mesh objects. To display links, first select the linked objects. On the Display panel 
Link Display rollout, turn on Display Links to see the links. You can also turn on Link Replaces Object to see only the links and not the objects.