|It's pretty tough to get it wrong. Click on the links to see pics of the connectors.|
The main power to the motherboard is supplied by either a ATX 20 pin main power cable or ATX 24 pin main power cable & there should also be a square 4 pin ATX +12 volt power cable that connects to the board in the vicinity of the processor.
Hard drives & optical drives are powered by either a 4 pin peripheral power cable or an SATA power cable.
The floppy drive (if you have one) is powered by a floppy drive power cable.
If you have a video card, it *may* require an aux power connector. Sometimes it's a 4-pin peripheral or floppy drive connector, but it could be a 6 pin PCI Express power cable.
Considering the varying sizes & shapes of the plugs, it's virtually impossible to do it wrong. There are almost always several extra plugs available when you're done, that's normal...they're for future upgrades.