When editing a model with the transformation tools, the Symmetry Settings lets you make changes symmetrically.

To transform components with symmetry
You can highlight components before you actually select them. As your cursor moves close to a component, a colored highlight appears on the nearest component. See Highlight components before selecting them. The transform manipulator snaps to the selection and the component(s) on the opposite side of the mesh are also selected.
The corresponding component(s) on the opposite side of the object are also transformed.