Three (soon to be four) types of credit-card-sized cards plug into portable computers (and some desktop models) to add and remove RAM, modems, network adapters, hard disks, and other devices without requiring that you open the box. These PC Cards conform to several standards set by the PCMCIA.
The original Type I PC Card is 3.3mm thick, a format used mainly to add RAM. Type II cards are thicker (5.0mm) and often are used for modems and LAN adapters (though they're also used for RAM). Type III cards are much thicker (10.5mm) and often are used for hard disks and radio devices.
All PC Cards use a two-layer interface of instructions: card services and socket services. Card services manage the system resources the card requires, and they determine IRQs and memory addresses on PCs. Socket services communicate directly with the PCMCIA controller chips and act as a go-between with card services. Socket services can be part of the BIOS or managed via software.
See also: PCMCIA