Well... i can just give you a basic walktrough on com (if im making a mistake people, please correct it for me :) )

COM (Component Object Moddeling) is a nice technology. its one of the easiest ways to update a program. Under direct x, com works like this: Directx is updated to work better, get bugs out excetra. Problem is that directx is 500mb+ and noone wants to download 500mb to get the new c version. thats what com is used for. instead of sending the whole program again, it sents the part that is edited. which makes it a lot smaller download

now about the interfaces. well, sometimes you dont WANT to work with the new stuff (especially since directdraw and direct3d are merged under directx graphics). thats what interfaces are used for. when you call an interface, you actually specify what version you want. simple said: if you dont want to work with version 8, you can work with version 7 of directx. the inferface can be called the version of the program you want to use.