Photoshop CS is a software for Image-Editing, manipulation, etc.
ImageReady is has features for image editing, but it is mainly used for optimising your graphic for the web and such.
In my experience, Photoshop CS and Imageready always come together. I don't think there is a separate install for either of them.
Photoshop is obviously better to make crazy effects, and basically do whatever with your images, while Imageready is better to optimize for web, making the file sizes small, etc.
Hope the explanation was enough.