I think the only difference is that ImageReady is made more for web graphics like buttons and simple interface componants. It has a slight bias towards working with primative shapes rather than complex images.
That said they are both based on the same technology and probably share most of the same code. This may be why Adobe don't seem to clasify them as seperate products, you might be able to get Photoshop by itself but I don't think you can get imageready without photoshop.