Difference

PDF/A is a standardized class of PDF. PDF itself is not based on a standard. Standards are important for long-term preservation, and therefor PDF/A was developed.

Main focus lies on the usability in the future, so it's not possible to include audio- or videofiles in a PDF/A. Such files would require additional software, and it cannot be guaranteed that that software is available in the future. 

Because of the same reasons, the use of external ressources is also not intended in PDF/A. All parts (like fonts or images) must be embedded in the document. It's also not possible to encrypt a file, because that would make it harder to use the document in the future.

Creating PDF(/A)-files

It's possible to create a PDF with the most common Word Processor Programs via the "Save as..."-Button. In some cases you can choose between PDF or PDF/A.

A tutorial for creating PDF/A-files can be found on the website of the Vienna University Computer Center (zid.univie.ac.at/computer-rooms/anleitungen/pdfa-erstellen/)

A tutorial for exporting a QuarkXpress Layout as PDF is available at the QuarkXpress.helpmax page: (http://quarkxpress.helpmax.net/de/die-ausgabe/exportieren-von-layouts/exportieren-eines-layouts-im-pdf-format/)

https://www.pdfa.org/

http://www.differencebetween.net/technology/software-technology/difference-between-pdf-and-pdf-a/