A PCI bus card fits inside a normal sized computer tower and plugs in inside the tower. A PCMCIA card is a small card that plugs in to a slot in the side of a laptop and is about as wide as a business card.
PCI cards are used to provide add on functionality to a desktop computer and can provide improved graphics, audio ,wireless networking and other functions
PCMCIA provide similar functions but are used mainly with portable computers and are easier to install as do not require the computer to be opened up.
PCI slots are found on computer desktop motherboards, and PCMCIA slots are found on laptops. In order to access a PCI slot, the computer needs to be off, the case must be opened, and then a PCI card can be inserted. By contrast, PCMCIA slots allow for insertion of an expansion card without powering down or opening the laptop case.
PCI is a bus inside a PC which accepts permanent cards such as sound, graphics etc. PCMCIA is a standard for temporary cards say for accessing internet through a mobile phone, which are plugged into the side of a notebook. PCMCIA cards can be used in a regular PC with PCI bus using an adapter available easily.
One is used in desktop computers (PCI) and the others is used with laptop computers (PCMCIA). PCI stands for Peripheral Component Interconnect and PCMCIA is a form factor. PCI cards go into white slots on a computer motherboard and the PCMCIA cards slide into an open bay on the laptop, usually there is an eject button next to it.