GCSE stands for "General Certificate of Secondary Education" and is awarded for attainment (after sitting exams and/or coursework) in a specific subject (or subjects).
They were introduced in 1986, with the first sitting in 1988. They replaced O-Levels and CSEs and removed the need to have to choose between which of those qualifications to sit.
A grade of C or above is considered to be equivalent to an O-Level.
They are normally studied over 2 years, by 14-16 year olds (Year 10 and Year 11) but can be studied over a shorter period, or earlier
IGCSEs (International GCSEs), is as the name suggests, the international version of GCSEs. Many IGCSEs have reduced or no coursework element, for that reason, they are sometimes studied by private candidates as an alternative to GCSEs.