A "Rating" has a validity period and has to be revalidated i.e. an annual number of take offs and landings.
A "Qualification" is valid as long as the licence to which it is attached.
All UK night ratings are now maintained in accordance with JAR-FCL effectively making them Night Qualifications.
There is a currency requirement for the carriage of passengers; one of the 3 take-offs and landings must be at night, unless you hold a valid IR.
The privileges of both the rating and qualification are the same, valid worldwide.