This is a summary from one college:
You choose six subjects in the IB Programme
Three at Higher Level (approximately equal to A Level) and three at Standard Level. You must study one subject from each of the following groups:
English: World Literature
Foreign Languages: Choose either French, German, Spanish or Spanish (ab initio)
Humanities: Choose one subject from: Economics, Geography, History or Psychology
Sciences: Choose either Biology, Chemistry, Environmental Systems or Physics
Mathematics: Choose either the Higher or Standard Course or Mathematical Studies (for non specialists)
Choose either Art or Music or one other subject from Biology or Physics (for those wishing to take two sciences) or French (for those wishing to take two post GCSE languages) or History (for those wishing to take two humanities)
My son has the option to do IB in Sept but
a) his college doesn't offer Economics or Politics (which he really wants to do) and
b) he doesn't like the Physics syllabus
(and c) he doesn't particularly want to continue with maths or English )
so he is probably going to do 4 A levels instead (Politics, History, Economics, French) which is a shame as the extra bits of the IB would be brilliant for him:
The IB Diploma programme also offers special features in addition to your chosen studies
You will study Theory of Knowledge (ToK) which helps you develop your skills of rational argument and stimulate critical reflection upon the knowledge and experience you have gained both in your studies and generally in your life.
You will also produce an Extended Essay which offers the opportunity to investigate a topic of special interest in greater depth. The Extended Essay is seen by universities as a great strength of the IB Diploma because it introduces the kind of independent research and writing skills expected at universities.
Creativity, Action, Service (CAS) is an important part of the Diploma curriculum, which recognises the importance of personal development outside of academic work. Credit is given for skills, talents and participation in activities such as sport, theatre, community service and voluntary work.
However, it is the first IB year at the college he is going to, so having read the coverage of the "guinea pigs" in the first post I'm thinking he may still be better off doing A Levels (& having more spare time to pursue other interests, which he probably wouldn't have doing IB as it is pretty full-on by all accounts)