The qualifications that most universities accept from home students are A-Levels. (Stands for Advanced levels, exams sat at the age of 18, and historically the complement to the O-Level or Ordinary Level, sat at 16. The O-Level was phased out in the late 80s I think though, and replaced with the GCSE, or General Certificate of Secondary Education.)
Anyway, most people will take 3 subjects at A-level, some will take 4, and the odd one or two (and they usually are odd!) take 5.
A-levels are graded A, B, C, D, E and U. Obviously A is better than E, and U pretty much means you got no marks.
Universities therefore as a rule will make an offer of three A-level grades, say AAB in any subjects. Sometimes they may also specify that one of the higher grades must be in a certain subject, eg mathematics.
To further confuse the issue, there is also the AS-level, or Advanced Subsidiary level. These are taken the year before A-Levels and count as half an A-level, in addition to making up the first half of the A-level course. Most people take four AS levels, and drop one, carrying on with 3 to the full A-level. The marks you get at AS contribute 50% towards your A-level mark. They're usually represented as lower case letters, ie a, b, c, d, e, u. For example someone's total marks at A-level may be BBDb.
Some universities don't make offers by grades, but by UCAS points. (UCAS is the Universities and Colleges Admissions Service, the body through which undergrads have to apply for universities in the UK). Basically, an A is worth 120, B 100, C 80, D 60 and E 40. AS levels are worth half, so the above marks would make 310. Most of the 'good' unis ask for grades though rather than points, I suppose because it ensures quality over quantity.
Er, that was probably a bit long, but I think it'll give you the gist of it!