An unlocked cpu means you can set the cpu multiplier to any number you want. The cpu operates at the Front Side Bus (FSB) times the mulitplier. FSB is 200Mhz at stock times the multiplier equals cpu speed. ex: 200x17 = 3.4Ghz cpu speed. Please note that though you might be able to put in a high value, most phenom II's top out about 3.8ghz on stock volatages and 4.0-4.2 if you tweak the system variables well. Pick a multiplier too high and the system won't boot.
Overclocking boards also let you adjust vaious other multipliers and voltages to support your overclocking attempts.
In a locked processor you can not change the multiplier though often you can lower it but not raise it. To overclock these processors you need to change the FSB which raises the speed of other things that get their timing from it like RAM speed, PCI/PCIe Speed, HT Link speed. Overclocking boards allow you to change the mulitplier on these to help keep them closer to stock to avoid problems as well as adjust various voltages to get things to work.
...the Black Ed's are for tweakers and tweakers have accidents and accidents lead to high numbers of RMA's and that means that BE's are more expensive, but more expensive parts compete less well in the market place so they are supplied as a niche product only.