GCSE and IGCSE are certainly very similar. Both qualifications are at the same level: they are designed to test the completion of the UK National Curriculum's "Key Stage 4". They are usually sat at the end of UK Year 11, in the year a student becomes 16. They have no age restriction, though - and have been sat by many students younger and older. By most higher education institutions and employers, they are seen as equivalent qualifications.
The ‘I’ in IGCSE stands for International; IGCSEs were conceived to be more relevant to students learning in an international or non-UK context.
Their key differences are as follows:
- Access: GCSEs can only be sat in the UK, whereas the IGCSE is examined across the world including the UK.
- Choice: GCSEs and IGCSEs offer different subjects. For example, IGCSEs are not available in Latin or Ancient Greek.
- Timing: GCSEs are sat annually in May/ June, although Mathematics and English Language can also be sat in November. IGCSEs are available in November and January (in some subjects) in addition to May/June each year.
- League Tables: IGCSE marks are not recorded in UK GCSE League Tables.
Other perceived differences:
- Coursework: IGCSEs tend to have much less coursework than GCSEs. Historically this was true, however, under Michael Gove’s reforms to the GCSEs, coursework elements have been removed from many subjects.
- Difficulty: IGCSEs tend to be more challenging than GCSEs. Since their launch in 1988, their content and standard have remained challenging, and have not been subject to the same pressures to become easier that have marked regular GCSEs. However, since the reforms it is widely agreed that GCSEs and IGCSEs are of a similar level and that one is not significantly more challenging than the other. There is even suggestion by some that IGCSEs are ‘easier’ than GCSEs (link) although this is a contested view.