What is a "GCSE"? I recently read about it...?

It is the General Certificate of Secondary Education. In the UK you usually take the exams at the age of 16. It is typically between 7 and 10 subjects. Everybody has to take English language and Mathematics and then there is usually some choice of other subjects.

After that you either leave school or go on to the sixth form for "A" level exams. Usually it is just 3 or 4 "A" levels at the age of 18 and then you either leave school or go on to University for a degree.

Before GCSE exams they were known as "O" level exams for the "ordinary" level and the "A" levels were "advanced". The GCSE name has replaced "O" levels, but the "A" level name has stuck.

The sixth form name comes from the days when senior school started at age 11 and the numbering system started from "1st form" again, so people in the 5th form would get their "O" levels and either leave or go on to 6th form.

These days we start the counting at year one in a primary school and just continue the numbers through secondary education, just as the Americans do, but the name 6th form has stayed around by tradition.