Adobe Illustrator Artwork was developed by Adobe Systems for single page vector based drawings in either EPS or PDF formats. The AI format is a strictly limited simplified subset of the EPS format. It is the standard format used when saving out Adobe Illustrator files.
Earlier versions of the AI format are restricted syntax that conform to DSC's Open Structuring Conventions. It was originally a native format called PGF. In order to make PDF files compatible can be achieved by embedding the PGF data within the saved PDF file. This same technique is used when saving EPS compatible files in recent versions of Illustrator.