If you've ever opened a PC case and stared inside, or looked at a bare motherboard, you may be taken aback by the number and variety of connectors, pins, and slots that exist on a modern PC motherboard. In this guide I'll identify some of the most common (and a few uncommon) connectors on motherboards used in most home PCs. I won't cover server- or workstation-class boards here, just what you might find in a typical midrange or high-end home PC.
For a similar discussion of the ports that you're likely to encounter on the exterior of a PC case, see "Multiple Ports on Your PC: What Do They Do for You?"
Since no single motherboard contains every type of connectors, I've used photos of four different boards to illustrate key examples. In one or two instances, there is some overlap; but for the most part, connectors are mentioned only once. Many of them may exist across different motherboard designs, however.
Asus P5WDH Deluxe
Let's start with an older motherboard, an Asus P5WDH Deluxe. This motherboard has a few connectors that aren't included on current-generation boards, as wll as some that do are still included, but are more readily visible here.
Audio front panel: This ten-pin connector links to the front-panel headphone and microphone inputs. The particular connector shown is an AC97 connector, which existed prior to multichannel HD audio. It's still in common use today.
Azalia digital audio header: You rarely find this connector, used to tie the motherboard to multichannel digital outputs on the case, on current motherboards.
Serial-port header: This connector isn't physically present on the board shown--you can just see the solder points for it. But this header does appear on a few modern boards. It supports a nine-pin, RS-232 serial port, usually as a bracket that occupies a slot space on the back of the case. A number of RS-232 connections remain in use today, mostly in point-of-sale devices or specialized test instruments. Consumer boards typically don't have them.
FireWire (IEEE 1994a): Once common as a digital camcorder interface, FireWire has largely been supplanted by USB, and the motherboard makers are gradually phasing it out. Some professional audio hardware still uses FireWire, though; you may also occasionally find higher-speed IEEE 1394b headers, but they are even rarer.
USB 2.0 front panel: These connectors are used to link to the front-panel USB ports on PC cases.
SATA connectors: These components connect via cables to various storage devices, including hard-disk drives, solid-state drives, and optical drives.
IDE connector: Rarely found today, IDE connectors were used to link to older hard drives. In addition, until a couple of years ago, many optical drives supported IDE. Today, all new storage devices ship with SATA.
Floppy disk connector: The venerable 3.5-inch floppy disk drive survived for nearly two decades--an eternity in the tech universe. But unless you have a pile of old floppies, you won't need a floppy drive. And if you do find yourself needing a floppy drive, you can always pick up an external, USB-connected drive.
Now let's examine a more recent motherboard: an Intel D67BG, based on Intel's P67 chipset and supporting LGA 1155 CPUs (like the Sandy Bridge-based Core i7-2600K).
DDR3 memory sockets: Current-generation PC systems use DDR3 memory, but in many instances they support different operating speeds. The P67 chipset used in this board maxes out at DDR3-1600, but to achieve that level of speed you'd have to overclock the chipset--officially the P67 supports only DDR3-1333. Here, we see four memory sockets. The system supports dual-channel memory, meaning that the system is populated with paired memory modules, which are mounted in sockets of the same color.
CPU fan header: This connector is specifically designed to link to the CPU cooling fan. The system BIOS monitors CPU cooling fan speeds; and if the fan isn't connected to this header, you may get an error at bootup.
Eight-pin ATX12V (CPU power) connector: Back when the Pentium 4 processor first shipped, Intel realized that high-performance CPUs needed their own source of clean, dedicated power beyond what the standard 24-pin power connector could deliver. Thus was born ATX12V. You'll see four-pin connectors on lower-end boards supporting CPUs with lower thermal design power (TDP), but the eight-pin version of the connector is used with higher-end processors and on boards that users may overclock.
Power for secondary fans: Many motherboards with secondary-fan power headers; these connectors are mainly used to power and monitor various case fans.
PCI Express x1 connector: PCI Express is a serial interface, though multiple lanes may be ganged together. The "x1" refers to a slot supporting a single PCI Express lane; it is used for I/O devices that don't require bidirectional bandwidth greater than 500 megabytes per second (gen 1 PCIe). Sound cards, for example, are typically PCIe x1 devices.
PCI Express x16 (graphics): PCI Express x16 slots are used mostly for graphics cards, though they can be used with any PCI Express card. Confusion may arise, however, because not all PCIe x16 slots are true PCIe x16. Occasionaly, you'll see PCIe x16 connectors that are physical slots for accommodating graphics cards, but are actually eight-lane (x8) or even four-lane (x4) electrically.
On some boards, even slots that support true 16-lane PCI Express for graphics may revert to eight lanes if you install a second graphics card into a second PCIe x16 slot on the motherboard. The P67 chipset, for instance, has only 16 total PCIe lanes for graphics. So if you drop in two graphics cards to run in dual GPU mode, each card will have just eight lanes available to it. This situation isn't as bad as it sounds, though, since even eight lanes in a PCIe 2.0- or 3.0-based system delivers plenty of bandwidth for most games.
32-bit legacy PCI slot: The now-classic 32-bit PCI slot has been around since 1993. A host of expansion cards support 32-bit PCI; and to accommodate them, most motherboards are likely to have at least one 32-bit PCI slot going forward. You may see some system boards configured so that a particular back-panel case bracket can support either a PCI slot or a PCIe slot, with some overlap between the two because they're very close together.
Front-panel switch header: This header connects various wires to the front panel of the case, where they link to power and reset buttons, and status LEDs for power and storage-drive activity.
Next: Motherboards for AMD CPUs and for the Intel Z77 chipset