From BlenderWiki

Pivot Point

The pivot point is the point in space around which all rotations, all scalings and all mirror transformations are centered. You can chose among five general modes for your pivot points which can be selected from a drop-down list in the header of any 3D area, as seen here in (Pivot Point modes). Your job is to choose the most efficient type for the task and to position pivot point accurately.

• Active object
• Individual Centers
• 3D Cursor
• Median Point
• Bounding Box Center

Note that even if the above examples use meshes, the same rules apply for other types (curves, surfaces…) as well.