1. Front Panel Audio Connector. Most cases which have front audio jacks will have cables that plug in here. An elternative to this is a speaker/mic cable (green+pink) which runs through your case, and will plug into the rear audio ports
2. Optical Drive Audio Connector. These are very rarely used, usually all the data from your optical drives (CD/DVD/BlueRay drives) will be going down your SATA cable. Few drives will actually have the corresponding port on them
3. PCI lanes. These are expansion slots which can take cards that are on the PCI format. NIC cards, Sound cards, RAID controller cards, SCSI cards, USB/firewire expansion cards and even some video cards are some examples of cards that will potentially have PCI connectors. A PCI lane is NOT the same or compatible with a PCIe lane (19 & 21)
4. Front panel connectors. Your power and restart buttons and your power and HDD activity light cables connect to this. Usually the wires from the case won't connect directly into this, they will have a "block" which the wires plug into, which then gets plugged into the connector on the board. It doesn't change anything, it just makes it easier to get the wires in the right place, and to get them in in the first place
5. USB connectors. If you have front panel USB/firewire/eSATA connectors, they will plug into here. You can also get USB/Firewire/USB ports for the back of your computer which fit into the slots for your expansion slots, but they plug into these connectors, rather than the PCI/PCIe lane
6. CMOS battery. Your CMOS must have constant power to it, as it isn't flash memory. This means if power is cut off (the battery dies or is removed for a period of ~15+ mins), the CMOS will be cleared, and your BIOS settings reset
7. SATA connectors. Near enough all modern hard drives, Solid State drives and optical drives will use a SATA interface. They all plug into here. The different colours show which will work together to work in RAID with the onboard RAID controller, and which will not work together in a RAID configuration.
8. The BIOS chip. It can be removed, but really isn't advisable at all, it contains all of the data for your BIOS
9. IDE port. Slowly becomming redundant, it was the industry standard before SATA was. There are still some optical drives that use an IDE interface, but very, very few Hard drives will now
10. Fan header. Your 3 pin fans will plug in here
11. 24 Pin power connector. Some boards will have 20 pin rather than 24, and most power supplies will have a 20+4 pin connector. This means that the motherboard connector has only 20 pins, but there is an extra 4 pins that can snap onto the end to make it a 24 pin connector. This makes it compatible with all motherboards.
12. See 10
13. Memory slots (DIMM slots). Your sticks of memory will go here. They will only go 1 way (notice the indentation) and will only support 1 standard of memory (again, notice the indentation). The colours show which DIMM slots work in dual channel together, with matching colours working together.
14. 4 pin fan header. The difference between 4 and 3 pin, is that 4 pin connectors come with fans that have variable fan speen control, meaning they take outputs from temperature sensors on the CPU or motherboard (depending if it is a 4 pin fan header, or 4 pin CPU fan header), and will increase speed if the temperature increases, or decrease if the temperature decreases, to reduce power consumption and noise production
15. CPU socket. Your CPU goes here. You never need to push a CPU into the socket, so when building DO NOT TRY. Only certain types on processors will fit in certain sockets, but just because they fit, does not mean all will work, it depends what wattage processors are supported, and in some cases, what northbridge and/or BIOS version is on the board.
16. CPU power connector. This provides extra power the CPU, and can come in either 4 or 8 pin varieties. Again, most power supplies now will have 4+4 pin connectors, which work the same as 20+4 pin motherboard power connectors
17. See 14
18. Serial port connector. This will allow a serial port to be added to the system. Most systems won't have them on board any more, beacuse they aren't used very often as they are getting outdated, howeve a lot of boards will still have a Serial port connector (COM port) to allow for one to be included if needed.
19. PCIe x16 lane. PCIe is another expansion slot, like PCI, however much faster. All modern video cards are on PCIe interface, usually PCIe x16, though other expansion cards, such as NIC and sound cards, are also on this interface. The difference between the differenet PCIe lans (x1, x4, x8, x16 etc) is the length, and the speed. x16 is the longest, and provides the fastest speeds, x1 is the slowest and smallest. An x16 card will work in an x1 slot, and vice versa, though usually the ormer won't work because the plastic edge blocks the card
20. PCIe x1 lane. See 19.
21. See 19