From BlenderWiki

Copy Transforms Constraint

Description

The Copy Transforms constraint forces its owner to have the same transforms as its target.

Options

Target

This constraint uses one target, and is not functional (red state) when it has none.

Bone

If Target is an Armature, a new field is displayed offering the optional choice to set an individual bone as Target.

Head/Tail

If a Bone is set as Target, a new field is displayed offering the optional choice where along this bone lays the target point.

Vertex Group

If Target is a Mesh, a new field is displayed offering the optional choice to set a Vertex Group as target.

Space

This constraint allows you to chose in which space evaluate its owner’s and target’s transform properties.