PDF/A is an alternative to PDF meant to ensure more safety and durability for the content of your documents. The format includes an ISO standardization which eliminates the risk of modifications that might appear when the document is visualised in the future using different software.
In other words, PDF/A documents can ensure the document will be compatible with future software and will stay exactly the same in the years to come, as far as fonts, images, colours and many other details are concerned.