Berg Connectors are those white nylon power plugs/sockets that provide power to your HardDisks, CDROM drives, floppy drives, and other PC components needing power.
Sometimes expansion cards need more power than the bus can provide. For example, FireWire cards and SATA host adapters need to be able to power the external drive or unit, and taking large currents off the bus can cause system instability.
I thought they were called Molex connectors? -- 
CraigBox
And this page says I'm right: 
Molex disk drive power connectors
