A native format, in the context of software applications, refers to the file format which the application works with during creation, edition or publication of a file. In turn, a foreign format is not worked with directly, even though it may be supported by an application.
Example
A document writer application may support a multitude of files, ranging from simple text files that only store characters and not font faces or sizes, to complex documents containing text effects and images. However, when these text files or documents are opened, they are not necessarily edited in their original format.
Instead, the document writer may first convert the file into its own native format. Once the file is done being edited, the application will then convert the file back to its original format.
In some cases, applications may be able to open (import) files, but not save (export) them in the same format. This may be due to licensing issues, or simply because the feature has not been implemented in the application's programming yet. However, the application will typically be able to save the document in its own native format or any of the other foreign formats it is programmed to export.
For example, Microsoft Office Word 2003 is able to open Windows Write (*.wri) files, but cannot save them. Instead it is able to save them in its native Word Document (*.doc) format or a number of other common formats.