Initially, a PC's slots were "dumb" devices, conceptually little different from a household wall socket, except with more conductors so the system could move 8 bits of data at a time (an "8-bit bus"). Early PCs had to be configured manually, usually by manipulating tiny hardware switches on both the motherboard and the add-on cards, and also via software settings. This wasn't too bad at first, but as PCs become more popular and more add-on devices became available, getting all the hardware to work together became somewhat of a black art.

Over time, the PC's slots and buses gained more connectors, capable of moving 16 and eventually 32 bits at once, and they also became software configurable. By the mid-1990s--and largely at the behest of Intel and Microsoft--PC BIOSes learned to cooperate with the operating system and vice versa so that "plug-and-play" add-in cards could be detected and set up more or less automatically. Windows 95 was the first mass-market PC operating system to support plug and play (PnP), and from that point onward, PCs began to become self-configuring.

But PnP got off to a rocky start. (Wags referred to it as "plug and pray.") PnP could work quite well on PCI-based systems (Peripheral Component Interconnect http://www.techweb.com/ encyclopedia/defineterm?term=pci ), but PCI was still a relatively new technology in 1995. Most of the installed base of PCs in the mid-1990s were either partially or entirely based on older bus standards such as ISA http://www.techweb.com/ encyclopedia/defineterm?term=isa or EISA http://www.techweb.com/ encyclopedia/defineterm?term=eisa. These standards worked less well, or not at all, with PnP.

Surprisingly, it's still possible to find systems today--even brand-new ones--with these older architectures http://www.google.com/ search?q=isa+motherboard. But fortunately, all the major mainstream vendors have now moved fully to PCI. Today, PnP works mostly as it should, and all modern operating systems support it. See, for example, Microsoft's PnP pages here http://www.microsoft.com/ hwdev/tech/PnP/default.asp or various Linux PnP how-tos here http://www.google.com/ search?q=linux+plug+and+play.

PCI technology hasn't stood still; it's evolved somewhat since its introduction. For example, in 1997, Intel introduced AGP--the Accelerated Graphics Port http://www.techweb.com/encyclopedia/defineterm?term=agp--which is basically a variant of PCI architecture designed specifically for high-speed graphics cards.

Other current PCI variants include Mini-PCI (used mostly in notebooks) and PCIx http://www.techweb.com/ encyclopedia/defineterm?term=PCIx (intended for high-bandwidth applications). See this page http://www.computerhope.com/ help/bus.htm for a good third-party overview of all major PC buses.

Although these standard forms of PCI are unlikely to vanish any time soon, the successor technology is already in development. It's called PCI Express http://www.intel.com/ technology/pciexpress/, and it's another Intel initiative, albeit with wide industry interest and support (http://www.pcisig.com/home).

Beyond Conventional PCI
Intel calls PCI Express a third-generation technology. (ISA was the first generation, and PCI the second.) In fact, the original name for the new technology was 3GIO, for "Third Generation Input/Output."