It saves the image without any EXIF data, and converts to 8-bit if the file is 16-bit. It probably does some other things also, but those are the main ones as far as I know.
It also allows you to adjust the file size of the image to pretty much exactly what you want it to be. so if a site like dpc wants a photo thats no more than 150 kb, you can do that with save for web
