AI, Adobe Illustrator File (.ai)
The AI Vector image file format
Developed by Adobe Systems for the Adobe Illustrator, Adobe Illustrator Artwork (AI) files represent vector graphics fitting on a single page. They are typically stored in the EPS or PDF format. If saved for further editing, Adobe Illustrator uses the file extension AI for saving these drawings. The AI file format is composed of paths that are connected by points, and not of image data.
Technical details of AI files
Originally, the native format of AI files was known as PGF - showing a close resemblance to PDF. Compatibility with the PDF file format was achieved when a entire copy of the PGF data was embedded into the saved PDF file. Likewise, this dual path approach is used in newer versions of Adobe Illustrator when EPS compatible files are created and saved. AI files are saved as vector images. This means that the image quality is not getting lost if the user enlarges the image.