The ExpressCard slot is a PC Card slot, that is not backwards compatible with the 32-bit CardBus or 16-bit PCMCIA standard. On some models, this slot can be determined by an "X" on the blue eject button, differentiated from the concave "O" on the PC Card eject button.
There are 2 physical formfactors, ExpressCard/54 and ExpressCard/34. The numbers refer to the width of the ExpressCard in milimeters.
Electronically ExpressCards can be implemented as either PCI-Express or USB.
ExpressCard/54 slots can accept ExpressCard/34 cards, but not the other way around.
ExpressCard adapters are handled either by the Linux PCI subsystem as hotplug PCI-Express adapters, or by the USB subsystem (depending on the adapter card).
Models featuring this Technology
ExpressCard/34 (using an adapter)