unk is a special type that, like any, allows any value. Unlike any, however, unk only allows a very limited set of operations which are safe on all values. In other words, unk is the type-safe counterpart to any.
A value of type unk can only be assigned to a location of type unk or any.
any is a special type that, like unk, allows any value. Unlike unk, however, any allows you to perform any operation on the value, even if the type checker would otherwise be able to prove that a given operation is incorrect.
The type of any operation on a value typed as any is also any. A value of type any can be assigned to a location of any other type.
unk should generally be preferred over any wherever possible.