Parts of a computer motherboard

Computer Motherboard 

1. Back Panel Connectors & Ports

Connectors and ports for connecting the computer to external devices such as display ports, audio ports, USB ports, Ethernet ports, PS/2 ports etc. See image below for a close-up view.

2. PCI Slots

PCI: Peripheral Component Interconnect
Slot for older expansion cards such as sound cards, network cards, connector cards. See image below for a close-up view.
Have been largely replaced by PCI-Express x1 slots 

3. PCI Express x1 Slots

Slot for modern expansion cards such as sound cards, network cards (Wi-Fi, Ethernet, Bluetooth), connector cards (USB, FireWire, eSATA) and certain low-end graphics cards. See image below for a close-up view.

4. PCI Express x16 Slot

Slot for discrete graphic cards and high bandwidth devices such as top-end solid state drives. See image below for a close-up view.

5. Northbridge

Also known as Memory Controller Hub (MCH).
Chipset that allows the CPU to communicate with the RAM and graphics card.
Beginning from Intel Sandy Bridge in 2011, this motherboard component is no longer present as it has been integrated within the CPU itself.

6. CPU Socket

Insert CPU here. To learn how to install a CPU

7. ATX 12V Power Connector

Connects to the 4-pin power cable of a power supply unit which supplies power to the CPU.

8. Front Panel USB 2.0 Connectors

Connects to USB 2.0 ports at the front or top of a computer case. See image above for a close-up view.

9. Front Panel Connectors

Connects to the power switch, reset switch, power LED, hard drive LED and front audio ports of a computer case. See image above for a close-up view.
For more details on the individual front panel ports, 10. IDE Connector connects to older hard drive disks and optical drives for data transfer. See image above for a close-up view.
Have been replaced over by SATA connectors (see motherboard component #13 below).

11. CMOS Battery

Supplies power to store BIOS settings and keep the real-time clock running. See image above for a close-up view.
The CMOS battery found on most motherboards is the CR2032 lithium coin cell.

12. Southbridge

Also known as the Input/Output Controller Hub (ICH).
Chipset that allows the CPU to communicate with PCI slots, PCI-Express x 1 slots (expansion cards), SATA connectors (hard drives, optical drives), USB ports (USB devices), Ethernet ports and on-board audio.

13. SATA Connectors

Connects to modern hard disk drives, solid state drives and optical drives for data transfer. See image above for a close-up view.

14. Fan Headers

Supplies power to the CPU heat sink fan and computer case fans. See image above for a close-up view.

15. RAM Slots

Insert RAM here. To learn how to install RAM

16. ATX Power Connector

Connects to the 24-pin ATX power cable of a power supply unit which supplies power to the motherboard.

17. mSATA Connector

Connects to a mSATA solid state drive. In most cases, this SSD is used as cache to speed up hard disk drives, but it's possible to re-purpose it as a regular hard drive.

18. Front Panel USB 3.0 Connector

Connects to USB 3.0 ports at the front or top of the computer case.

19. Power & Reset Button

Onboard button to turn on, turn off and reboot the computer.
This motherboard component is more common among high end boards.