The 3D Cursor (sometimes called the Tri-view Cursor) is used indicate an exact position in 3D space. The intersection of any two 3D cursors in the Working-Volume Top, Front, or Side views indicates an exact position in 3D space. The 3D Cursor is positioned with the Windows' Mouse Cursor.
The 3D Cursor can take the form of either a Cross Hair or a Cross Wire. Note you can toggle between these two 3D Cursors with the F9 Key.
Cross Hair 3D Cursor
Cross Wire 3D Cursor
