The GCSE (General Certificate of Secondary Education) is the name of a set of British examinations, usually taken by secondary school students in England, Wales and Northern Ireland (but not Scotland) at age 15-16.
A different examination is taken for each area of study, but school students are usually obliged to take examinations for certain "core subjects" (English language, English literature, mathematics, and science) along with several optional subjects; sometimes up to 10-14 in total. There is also an option for students to take "short" or "half" courses for certain subjects.
There are different tiers for most examination - normally "foundation" and "higher", and for some subjects, "intermediate". Students are entered for a certain tier based on their ability. The tier a student is entered for affects the range of grades that student could attain.
It was introduced in 1988, and replaced both the O-level GCE (Ordinary level General Certificate of Education) and the CSE (Certificate of Secondary Education) qualifications.
Recently introduced is the VGCSE system, the "V" meaning "vocational," which points courses to the business and "applied knowledge" side of courses. VGCSE courses now include ICT and Manufacturing, amongst others.