For Express Cards. These are rather like the old PCMCIA cards (PC Cards) used in slightly older laptops. Both are ways of adding ports and external devices to a laptop. I have an Express Card with 4 USB2 ports for example. You can buy many different types - RS232 Serial Ports for attaching old serial devices and others.
Express Cards are smaller than PCMCIA cards and have some technical advances over the older system. They are NOT interchangeable. It is possible to buy an adaptor for PCMCIA to Express Card but the result is a very big and not pretty!
In some ways an external Docking Station offers many of the features of Express cards and is an all in one package. I use the latter when travelling but my Docking Station back at base.