From personal experience, IGCSEs are meant to:
(a) make it easier for students who have NO previous experience in a subject to catch up, so that students from poor educational backgrounds have a fair chance;
(b) give a standard that's acceptable for most international qualifications, so they can go on to do IB or A Levels or whatever;
(c) provide options for all schools, even if they have limited resources (flexible teaching scheme) and have accessible exams for homeschooled students and non-exam centre students (alternatives to practicals and coursework).
In short, the subject matter's larger, the teaching is so flexible that self-studying is feasible, and the syllabus are designed so that most countries will accept them. BUT the grade boundaries are also very flexible. The syllabi are a little different from the GCSEs, and they include lots of simple DIY practicals, but there really isn't that much of a difference. We use GCSE study guides, textbooks... Some people might find it harder, some easier, but in my opinion, they just help people in different conditions and from different Primary School systems to sit a standard exam
They really are quite awesome, and maybe a step up from GCSEs for foreign languages, a
little more application-oriented in Sciences.
Oh, and obviously, they're more 'international minded'—case studies, Lit, etc. But that's about it. (Sorry for the super long post, BTW)