The GCSE, standing for General Certificate of Secondary Education, is an academic qualification in a specific subject. It is gained at English, Welsh and Northern Irish schools in years 10 and 11 (ages 14-16). As of 2018, teenagers must stay in education, employment, or training until 18. Scotland has a different system.
GCSEs replaced the O-levels in 1988, with the idea being an integration of "The National Curriculum". Essentially, every child is taught the same things at roughly the same time. It gets more complicated, however, since there are half a dozen different examining boards each with their own set of GCSEs — a Science certificate from the AQA may require slightly different work than an Edexcel one, leading to some people selecting courses based on the perceived ease of exams and/or coursework. One does not have to follow one board across all subjects; doing an Edexcel ICT (Information Communication Technology, i.e. computers) course and an AQA Music course, for example, is quite common.
The GCSEs are graded under a new system 9-1, with 9 being the highest and being the equivalent of a top level A* (9-7 are about equivalent to A*-A, 6-4 are B-C, and 3-1 are roughly the same as D-G. U remains the lowest grade). The new GCSE system has removed, in most cases- in some subjects it is only reduced, controlled assessments where students would do a piece of formal work under exam conditions which would then be marked by a teacher and submitted as part of the final grade. Controlled assessments have likely been removed due to the discrepancy in marking between different teachers and that they are easy to cheat/liable to exploitation. The government is also attempting to phase out coursework and practical exams, and they have made it clear that the less study leave there is, the better.
There is also the lesser-known iGCSEs, which, as the "i" implies, are "international". iGCSEs generally have a wider curriculum, are accepted in more places across the world, and some UK-based schools are starting to use them. Currently, only two exam boards provide iGCSE courses; Edexcel, and Cambridge.
The choice of coursework and exam follows two distinct levels (where such a choice is available). These are "Higher" and "Foundation" Tiers (previously called "Advanced" and "Standard"). If you take a Higher/Advanced course, your possible grades will be 9-4 or a U. You cannot attain an 3, 2, or 1 grade in the advanced tier. If you take the Foundation/Standard course, your possible grades will be 5-1 or a U. If you do the standard level course perfectly, your highest award will be a 4 grade. Which paper a student will do is generally decided between them and their teacher, though depending on the school all pupils in higher-ability classes may do the Higher course.
To continue onto most Sixth Form Colleges, and for all Universities and skilled work, the minimum GCSE requirement is to have achieved the English Baccalaureate. This is 5 GCSEs at 9 to 5 grade in English, Maths, and at least one other science and humanity. To continue a subject at A-Level, most students have to achieve a 6 or 7 at minimum in that specific subject.
Back in the '90s, people would joke that you would get some sort of pass grade if you could correctly write your name on the paper. However, in 2012 a number of schools began looking at legal action because of accusations of unfair marking (the grade boundaries were altered). Several MPs have called for the GCSE to be scrapped, but then that has been happening with regularity since 1988.
Generally, somebody who does not have a passing grade in GCSE Maths and/or English (or another Level 2 qualification, like a functional skills certificate) will find it very hard to find even a low-level job, and people who fail these often resit GCSEs in the autumn.