From BlenderWiki
Mirror
Mode: Edit mode
Hotkey: M
Menu: Mesh » Mirror » Axis corresponding to the wanted transformation orientation
The mirror tool in Edit mode is a somewhat enhanced version of the same tool in Object mode. It is exactly equivalent to scaling by -1 vertices, edges or faces around one chosen pivot point and in the direction of one chosen axis, only it is faster/handier.
Let’s see this in detail.
Pivot point
Pivot points must be set first. To learn more about pivot points, see this page. Pivot points will become the center of symmetry. If the widget is turned on it will always show where the pivot point is.
Transformation orientation
Transformation orientations are found on the 3D area header, next to the Widget buttons. They decide of which coordinate system will rule the mirroring. See their dedicated page for more details.
Axis of symmetry
For each transformation orientation, you can chose one of its axis along which the mirroring will occur.
As you can see the possibilities are infinite and the freedom complete: you can position the pivot point at any location around which we want the mirroring to occur, chose one transformation orientation and then one axis on it.
Here are three examples given to help figuring out what needs to be done and what results can be expected. In each case the whole geometry was duplicated with ⇧ ShiftD and the resulting copy was mirrored.
On (Mirror around the Individual Centers…) the pivot point default to median point of the selection of vertices in Edit mode. This is a special case of the Edit mode as explained on the pivot point page. The chosen transformation orientation is Global and the chosen axis is Y.
On (Mirror around the 3D Cursor…) the pivot point is the 3D Cursor, the transformation orientation is Local, a.k.a. the Object space, and the axis of transformation is X.
On (Mirror around an active vertex…) the vertex at the very tip of the spout is the pivot point by choosing the Active Vert/Edge/Face option and then A to select all vertices followed by RMB 
twice on that vertex to make it active. The transformation orientation is View and the axis of transformation is X.
In Conclusion
To summarize our survey of the Mirror tool a few recommendations:
- Do not mistake it for the Mirror modifier.
- Also remember that the results are the exact equivalent of a scale=-1 along an axis of the current transform orientation. The advantage of the Mirror tool over this is that it is faster to use and almost foolproof.
