The 8 pin connector is the same as the P4 connector with 4 more pins. This is the CPU power adapter.
I have a M3-ATX which has the 20 pin connector, a SATA power connector and an older style 4 pin HD connector.
You can buy adapters to go from the 4 pin connector to the 4/8 pin connectors.
I bought an 8pin to 8pin extension cable and took a 4 pin Y cable I had and combined the two. The 4 pin adapter has 2 grounds(black), 1 +5 (red) and 1 +12(yellow). The 8 pin connector has 4 +12 (yellow) and 4 ground(black). My manual says not hooking up the connector can cause the CPU to work erratically. If you are using a low power CPU and your power supply supplies a P4 you SHOULD be able to get by with just the 4 pin connector. It will only fit on your board in one way. Check your manual. My motherboard talked about 4 pins supported something like 65watt and lower CPU's and for higher you need the 8 pin connector.
So just check out what your manual actually states about it. When all else fails it doesn't hurt to provide all 8. I also bought a 20-24 pin converter to allow me to use all 24 pins but I haven't tried to use it. My CPU is a 55watt I3 processor.