One of the important differences between previous educational qualifications (and the earlier grading of A-levels) and the later GCSE qualifications was supposed to be a move from norm-referenced marking to criterion-referenced marking.  On a norm-referenced grading system, fixed percentages of candidates achieve each grade. With criterion-referenced grades, in theory, all candidates who achieve the criteria can achieve the grade. A comparison of a clearly norm-referenced assessment, such as the NFER Cognitive Ability Test or CAT, with GCSE grading seems to show an unexpected correlation, which challenges the idea that the GCSE is a properly criterion-based assessment. 
GCSEs taken in schools usually include a significant amount of graded ‘coursework’. This consists of projects, essays or other work done during the year which counts towards the final grade in the exam. This is ideal for students who find exams stressful. It can be encouraging to know that they have already achieved twenty to forty percent of the grade. However home educated students sometimes find it harder to manage coursework. This is because an independent person has to mark it. For this reason, some choose the IGCSE (International GCSE) exams which rely entirely on exams. This suits some students, but not others. You can read a lot more about IGCSEs, including information about several specific courses, at the Eddis Tutorial site .