To add to Ganesh's excellent answer:
The UK's education system doesn't offer the 'breadth' (in the post-16 years) which can be found elsewhere in the world. For that reason several leading schools in the independent sector (and a few bold schools in the state sector) were turning away from traditional A-levels, in favour of the International Baccalaureate. That resulted in a series of educational reviews, which decided that the best way forward was to retain A-levels but to offer more breadth in the first year of studies (and to offer a new AS qualification for that first, or only, year of study).
The increasing number of overseas students, together with a policy of encouraging applications from people with qualifications other than A-levels, meant that universities needed a standardised way of assessing an applicant's qualifications. So the UCAS Tariff was introduced. An AS-level 'A' grade and an A-level 'D' grade are both awarded 60 points, as is an 'A' grade from the Institute of Financial Service's Diploma in Financial Studies. Points are available, on a sliding scale dependent upon grades, for the International Baccalaureate, as well as for qualifications from Ireland, the USA, Canada and Hong Kong.
If an applicant studies the piano, up to 75 points are available for practical skills and 30 points for theory. (Grade 8 Distinction in both cases). An accomplished equestrian can gain 35 points for a Stage 3 pass in Horse Knowledge and Care. A further 35 points are also available for practical riding skills at Stage 3 level.
Full table:
http://ucas.com/stude..._tariff/tarifftables/
Chris