PCMCIA Cards come in a few distict types, mostly based on physical size,:
- Type I
3.3 mm thick. The first PCMCIA Cards, uses primaily for non-volitile memory and thus lacked any soft of real I/O Spec, so they were never used for anything more advanced than storage.
- Type II
5 mm thick in the middle, still 3.3 mm on the edges. The most common form factor today. The extra millimetre gave a bit more headroom inside, and they were granted a generously design I/O spec that included the mythical beast Plug and Play. Nearly every kind if device is available in Type II form, including some hard drives. Also allowed for use of 3.3v, 5v and 12v power.
- Type III
10 mm thick -- the same size as two Type II Cards stacked, but electrically identical to Type II. Mostly used for Hard drives and high-end network cards that provide 'real' plugs.
- Type IV
15 mm thick. Intended for large capacity hard drives, the standard was never ratified and thus is seldom seen. No Type IV devices exist and only a handfull of card slots are large enough to accomidate them.
It is interesting to note that, although everyone calls them PC Cards now, the name conversion from PCMCIA only technically applied to Type II cards. No one really cares, I'm sure, but 'PC Card' means Type II PCMCIA.
There is also another advanced segment of cards, called CardBus, that really deserve their own descriptions. | 
