A simple question but see how long the answers will be
It stands for 'international' and in very basic terms was taken up a few years ago by fee paying schools and was then accepted by government into league tables so state schools gave taken up, mainly science subjects and English.
It is generally seems as a return to more old fashioned approach - no modules ( this was before GCSE no module change) and generally more vigorous ( debatable of course!) For example, our local GS goes iGCSE in individual sciences as believe it to be better prep for A level, which many of the boys go on to do.
In the past it was considered harder to get the top grades than in GCSE but with changes to GCSE that may change anyway.
Lots of people have different opinions but hopefully this is an outline