If Photoshop were the answer to all your graphic problems, you wouldn't need a creative suite. Photoshop is best suited for making raster graphics for high-quality print output. Digital photo editing and manipulation is what this program is for at its core. Use the Brush tools to create original art, or mix original art with existing photos. Take many pictures and assemble them into a collage. Perform color corrections and photo repairs. Use ImageReady to take your finished Photoshop creations and prepare them for the web using ImageReady's advanced compression options and slicing tools.
Photoshop seems to do everything, doesn't it?
Photoshop certainly does a lot. But that doesn't mean it outperforms other applications in the suite. You want to use the best tool for the job. For example, Photoshop can make PDF files, but they save as pretty large files even with simple graphics. If you need to make light PDF files for sharing on websites, use InDesign or Illustrator. Although Photoshop has some vector drawing tools for making logos and other objects, you should be using Illustrator for that sort of work because its design tools for vector art are superior. If you are creating websites, you should use Photoshop in combination with GoLive. It's surprising but there are a number of websites out there that are simply Photoshop graphics and text sliced and embedded into a page. Use Photoshop for your interface design, but your text should not be rendered out of Photoshop. Websites should be made up of HTML text, not Photoshop graphic text.
ImageReady is meant to work with Photoshop as a next step in creating web graphics. Though the differences between the programs seem small, more options for saving and optimizing web graphics and animations are available in ImageReady than in Photoshop.