The difference is that an image can belong to any number of categories - and you can categorize a new image by just one mouseclick. You can then retrieve your images by the selected properties. You can maintain an unlimited number of these databases, eg. for different themes. Resample - You can resize an image using high quality resampling. Retriever does both things at once.
Retriever chooses the best things to do for the given image without ruining it.
Reduce the number of colors used by an image, using palette optimization and dithering. In addition to selecting part of an image with the mouse to get a new image, Retriever can cut images that contain multiple pictures, or pictures spoiled by a fancy frame automagically . This is useful for digital camera pictures and scans in landscape format. Unlike other viewer's full screen modes you keep having full control of the program, so with Retriever you can actually work in full screen mode. Even works for multiple files at once.