IGCSE stands for International GCSE, mainly taken by international schools but normal schools in UK also offer them sometimes.
The major board for IGCSE is CIE (Cambridge International Examination) and Edexcel.
IGCSE maths and sciences are said to be harder. I am not sure on exam difficulty, but I know for sure that IGCSEs have extra topics that aren't covered in normal GCSEs - I know this as I have done IGCSEs with CIE myself and I'm now at a school where some did IGCSE while others did GCSE. With French, IGCSE is harder due to its Extended Writing component.
As to Geography/History, unlike Geography/History in GCSE, IGCSE has no topic specific to UK. We only do world geography and history with IGCSEs. History is allegedly harder with IGCSE according to my previous history teacher, and it seems to be true looking at exam papers from other boards, and having done few of these papers as part of practice. I have never seen past papers of geography from other boards but I personally found IGCSE geography quite easy. The main paper (paper 1) in CIE IGCSE geography has 6 big questions and you get to choose 3 of them, with first 2 questions guaranteed to be Population and Settlement topics. So you can get away with only studying these two topics plus some insurance topics. I don't know how it works with Edexcel IGCSE or normal GCSEs - if it's not this way with other boards then I reckon CIE Geography is easier.