For most users point of view PCMCIA slots and express card slots are used for the same thing, they are not compatible with each other, so a PCMCIA card wouldn't work in an express card slot but the end use of them is essentially the same.
the most common thing I've seen PCMCIA slots used for is either wired or wireless network cards, but with new laptops having both built in that use has become largely obsolete.
they can also be used to add ports that your laptop may not have, such as a serial port, firewire port, parallel printer port or other items such as TV tuner cards or even another graphics card.
As I said, they are pretty much just a generic expansion slot.