Lets say you have an i5-4670K
3.4 MHz, turbo 3.8 MHz
Under light load(1 core being utilized), it will "turbo" (overclock) to 3.8
Same with two cores, but it will only overclock somewhere below 3.8
and less again with 3 cores, ect...
Now if you overclock, all cores will run at that OC underload. It offers the best boost, as under gaming you would likely be loading your CPU beyond 1 core. Plus generally you overclock well beyond the turbo speed. Say 4.4 MHz with the above mentioned i5.
It's worth noting, though, that changing the base clock speed (ie overclocking) beyond the turbo boost clock speed will not correspondingly increase the boost clock. In this case, the boost clock will be ignored. The boost clock is only applicable to processors running at "stock" speeds, essentially.