PCIMA is the real name of PC Card.
They can be anything; an Ethernet NIC, an analogue modem, a Wi-Fi unit, a Firewire card, a GPS unit, A video input card, etc...
They may be hot-swapable, I'm not sure. They can do everything that PCI does but because it's in a easier to access place than on a normal computer, it can be other, more useful things.
However, since USB got on the scene, it's been dying. Thats why it's only on the 15-inch and 17-inch powerbooks, because they are designed to have everything. It's faster than USB I think though, but USB is fast enough for most things.
I personally think that having one on an iMac or e-Mac would be useful, because thier designs don't allow normal, full sized cards. And it would allow better expansion. The new iMac may have one.