I’m sorry to mixture things.
Pdf/A is the ISO 19005-1:2005 standard, also known as format for the long-term archiving of electronic documents.
It’s a subset of the “general” Pdf format. And in this moment it’s in the 1 version, more precisely there are 2 sub-versions, Pdf/A-1a (the most restrictive) and Pdf/A-1b (the more “relaxed” version).
You can have more info here:
http://en.wikipedia.org/wiki/PDF/A.
Preflight is an option in Adobe Acrobat Creator where one can check if a file complies with a certain Pdf version (besides Pdf/A there are other subsets, like Pdf/X, etc…).
Pdf/A is very important to Archivists and Record Managers all over the world, as it can, and we (information professionals) are fighting to make it be, the “de facto” standard for migrating administrative and business documents (analogical and digital) and Archive them for the future.