Dear Theo,
Another exam question? 
Any answer involves a wide range of factors many of which can be seen detailed in this forum.
A short answer, I suppose, is that the the fundamental difference is in the name. Public standards are supposed to directly / indirectly satisfy a perceived public requirement. In practice, they seem to often result from demonstrated deficiencies in the existing setup, eg HACCP, the Farm to Table initiatives. Not so unusual of course. Political aspects are also suggested to be occasionally not unrelated, hence the various (some very long term) arguments involving GATT.
Many (all?) private standards are market driven, eg initiated by retailers like BRC which is closely linked to the legal requirements for due diligence, esp. in UK, although I suppose it can be (generously?) stated as also serving public consumer interest as a by-product. IFS has presumably a similar function in France and Germany??. All now under the benevolent umbrella of GFSI. Other EU countries maybe hv their national equivalents also although the fore-mentioned seem to dominate in EU?? SQF ?? (I always thought it was Australian but it now seems to have been taken over by the USA??). And somewhere in the middle is the neutral ISO 22000.
Rgds / Charles.C
