Portrait Preset Camera Mode

Portrait preset mode is most often represented by a silhouette of a head and is designed for taking "head shots". This means a head and shoulders portrait of a person. This mode is not well suited for full-length portraits or groups of people.
Portrait mode tells the camera to default to a:
-Slow Film Speed
-[linkurl=http://photography.about.com/od/takingpictures/ss/DOF_2.htm]Small Depth of Field (Large Aperture)

This is supposed to result in a fine film grain and blurred background. On some cameras, however, the default depth of field is so small that part of the subject's face becomes blurred. Also, the shutter speed can become slow enough to trigger the on-board flash.

Source : http://photography.about.com