The IPC standards are a collection of standards taken from the major players in industry, put together so that there are at least some standards available.
This is mainly for the US market, although some other countries do also use them because they may not have thier own.
AFAIK they are not mandatory, however if you wish to sell in the US market it may be, or at least more benificial to conform to the relevent ones.
UKPCBL may be able to explain better.
They are copyright documents although there are some floating around the web for download.
As an example std, the IPC782 SMT PCB footprint standard - I used that for my SMT libraries and it improved manufactoring because there were less faults, solder balls etc. Now that it has been replaced by the IPC7532 std that should be even better.
www.pcblibraries can explain better about that, with the free footprint calculator too.
Other standards define what is good & what is not for PCB assembly, laminate fabrication etc.
yes they do also include track clearances.
As for parralell standards, there are British standards available for free viewing if you are a member of a british library.
but I do not think that they are as good as the IPC ones as the BS ones only seem to be about symbol shapes.