PC Card or PCMCIA

Personal Computer Memory Card International Association (also: people can't memorize computer industry acronyms )

You use this slot, which is very important, to connect cards that are about the size of a credit card which gives you functionalities that you may need.

The PC Card Standard is very flexible, allowing the integration of an enormous variety of peripheral devices. Some of the applications which have been developed throughout the years of the existence of the PC Card Standard:

10/100 Mbps Ethernet Adapters
A/D Converters and other Data Acquisition Devices
AM/FM Radio Tuner
Biometrics cards (Fingerprint reader)
Bluetooth cards
CD-ROM Interface
Cellular Phone Interface (WAN)
Digital Camera
Docking Station Interface
Ethernet LAN Adapters
GPS (Global Positioning System) Cards
Hard Drives (Rotating)
Infrared Wireless LAN Adapters
ISDN Cards
Joystick Interface Cards
Memory Cards - Flash, SRAM, and many others
Memory Cards Adapters - SD, , MMC, SmartMedia, CompactFlash, MemoryStick, etc.
Modem/Ethernet Combination Cards
Modem Cards
Parallel Port Interface
PDA PC Card
Radio LAN Adapters
SCSI Adapters
Security Tokens
Serial Port Interface
SmartCard Readers
Sound Cards, Input and Output
Token Ring LAN Adapter Cards
TV Tuner
VGA
Video Capture/Frame Grabber Cards
Video Teleconferencing Cards

There are 3 types (sizes) of PC card slots:

Type I
The original PCMCIA specification cards (version 1.x) were Type I. These have a 16 bit interface. They were only used for memory expansion. They have a single row of connector pins and are 3.3mm thick.

Type II
16 or 32 bit, 2 rows of pins. 5.0mm thick. These have I/O support and so can be used for almost any sort of peripheral.

Type III
16 or 32 bit, 4 rows of pins. 10.5mm thick. These are used when extra thickness or bandwidth is important, e.g. storage device, Zip drive.