When to use the Flash.

Disadvantages:

• Give a harsh light to your subject, thus changing the true colors.
• Limited range and coverage.
  
  

  Harsh / Washout / Flatten	hp.com
  Flash used, harsh colors.	Flash not used, true colors.

When NOT to use:

• Do not use automatic flash mode.
• Do not use near mirrors or windows.

When to use:

• If you must use flash, use flash compensation to control for better lighting and colors.
• Light source is behind subject, causing shadows on subject, otherwise known as fill flash.
• Some newer cameras are smart enough to detect the scene below and automatically apply backlight compensation, so firing fill flash is not necessary.
  
  

  Fill Flash	danheller.com
  No flash, shadows on face.	Flash used, brighter face.

 

References:

• http://en.wikipedia.org/wiki/Flash_(photography)
• http://www.hp.com/united-states/consumer/digital_photography/take_better_photos/tips/flash.html
• http://www.shortcourses.com/use/using6-0.html