ADOBE - Illustrator, Fireworks and Photoshop

Can someone give me a brief explanation of the differences between these 3 programs and when I would want to use one over the other (maybe of an example of when I would use Fireworks versus Illustrator or Photoshop).  I see the master collection contains all 3 image editing applications. 

Fireworks was once a part of Macromedia correct?  Will these programs ever merge?