Just to add some specifics, if you look at one edge of the cards where they insert into the machine, you'll see there's a tab that works as a key, cut out like a rabbet joint. The one on a PCMCIA card has about 2/3 of the width removed. The one on a CardBus card has about 1/3 of the width removed, which is the physical device that prevents you from inserting it into older slots. PCMCIA uses a 16-bit bus, whereas CardBus uses a 32-bit bus, one of the major differences between ISA and PCI. (PCI can also use a 64-bit bus, but that's a whole different story.)