Answer:

An Activation Key (also known as a Registration Key) is a code that lets you use a copy of Foxhound that you have already installed.

Activation Keys are available for purchase here, and the Foxhound Activation page will tell you which kind of key you need... or you can look use the following table:

If ...	Then ...	With a ...
You have just installed a fresh copy of Version 4, without upgrading from an existing copy of Version 1, 2 or 3	You can activate it as the Rental Edition	30-day Rental Key
or ...	You can activate it as the Basic Edition	New Basic Key
or ...	You can activate it as the Extended Edition	New Extended Key
If ...	Then ...	With a ...
You originally installed Foxhound Version 1, 2 or 3 and you have now upgraded it to Version 4	You can activate it as the Rental Edition	30-day Rental Key
or ...	You can activate it as the Basic Edition	Basic Upgrade Key
or ...	You can activate it as the Extended Edition	Extended Upgrade Key
If ...	Then ...	With a ...
You are now running a Rental Edition copy of Version 4	You can extend or renew the rental period	30-day Rental Key
or ...	You can change it into the Basic Edition	New Basic Key
or ...	You can change it into the Extended Edition	New Extended Key
If ...	Then ...	With a ...
You are now running a Basic Edition copy of Version 4	You can change it into the Extended Edition	Basic To Extended Key

See also...
What are the differences among the different editions of Foxhound: Rental, Basic and Extended?