Vcore supplies the juice to cpu. By default socket 754 desktop cpus are spec'd to 1.5v vcore while socket 939 desktop cpus are 1.4v for 90nm winchesters and 1.5v for 130nm newcastle cpus. Increasing vcore allows cpu to overclock higher in terms of overall cpu raw speed. Increasing vcore also increases cpu temperatures and the amount of heat being generated by the cpu.
Some motherboard bioses also list as vcore VID, CPU VID control/VID special control etc. Some bios give straight vcore options, while boards like MSI and DFI give you straight vcore (VID Control values up to say 1.55v), then allow you to use CPU Special VID to add a percentage onto that base VID value.
For example, in DFI NF4 boards you can set vcore up to 1.55v via CPU VID control. If you set CPU Special VID control to "Above VID * 104%" you theorectically will get 1.55 + 4% = 1.612v vcore. Boards my undervolt, i.e. on DFI NF4 1.55 + 4% gives me 1.55-1.57v in windows.
There's a CPU VID Startup value on DFI NF4 boards too, I tend to just leave that as AUTO.
