PCI stands for P
It is a standard for device interface in a computer. It can operate at either 32-bits (with a 124 pin connector) or 64-bits (over 188 pins). Originally designed to replace VESA Local Bus in Fall 1993. Operates between 20-33 MHz (32-bit) and 66 MHz (64-bit and some 32-bit). To be technical, PCI is actually not a bus at all, but rather a bridge that allows asynchronous access between the CPU and slower peripherals.
PCI supports Bus Mastering, which is a standard developed to give a device temporary exclusive control over the rest of the bus. This is good for things like IDE controllers, where having control of the bus can allow good and steady streams of data from the hard drive to the memory or CPU.