I am developing a business application and, provided that many companies look for integration, I would like to make it "compatible" with business systems like SAP or Navision. What mechanisms do these systems use for importing/exporting/syncing data with third-party applications?
There exist software tools known as EAI (Enterprise Application Integration) whose purpose is to act as middleware to enable the integration of applications across a company.
Apache Camel is an example of such framework, but there exist many of them. You can find a comparison list here: http://en.wikipedia.org/wiki/Comparison_of_business_integration_software
As the user nmiranda pointed out, in the case of SAP, the framework used for data interchange is SAP PI (SAP NetWeaver Process Integration).
I think your question was actually aimed to find this "starting point", wasn't it? I faced the same question some years ago and I also wondered if there was any "standard" interface to integrate applications. In such case, I hope have helped you.