I've gotten many questions before asking about the difference between Photoshop and Fireworks. Why one over the other? Do I need both? Etc!
Well Photoshop is primarily a bitmapped (data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium) editing program where Fireworks is a vector (the use of geometrical primitives such as points, lines, curves, and shapes or polygon(s), which are all based upon mathematical equations) editing program. More than you want to know? In short, bitmapped graphics are not scalable where vector graphics are. Both programs can work with both formats but Fireworks is a program that is most beneficial to Web developers. One can use very specific tools to minimize file size (image compression), making files smaller in size while holding as much detail as possible. In addition Fireworks lets you create hot spots (links) right into the image and break larger images into segments allowing the image to load faster on web pages. HTML code is created automatically in the background and can be copied into the web designs you are building. Fireworks also exports the images segments you have selected into specific folder that can also be moved onto your sever space. These are just some of the advantages of Adobe Fireworks.