From wiki:
A whitelist is a list or register of entities that, for one reason or another, are being provided a particular privilege
A blacklist (or black list) is a list or register of entities who, for one reason or another, are being denied a particular privilege
From me:
So, if you want to stop everyone from registering except the UK (for example).. the you could either:
1) Just select the UK and select the white list options
or
2) Select everyone apart from the UK and select the black list option
What the white list option really does extra.. is that if an IP address (for what ever reason) can not be identified with a country code, then they wont be let through the registration
However, this could be regarded as a negative side of the white list (some legitimate people might not be able to register with this option).
It is safer (for legitimate users) to use the black list option
The white list option is there to make it easier to prevent registration from everyone (except a few), and also be "strict" about the registration (unknowns cant register)
The black list option is there to make it easier to prevent registration from a few countries, and also less be "strict" about the registration (unknowns can register)