An AI file is a drawing created with Adobe Illustrator, a vector graphics editing program. It is composed of paths connected by points, rather than bitmap image data. The file is commonly used for logos and print media.
Since Illustrator image files are saved in a vector format, they can be enlarged without losing any image quality. Some third-party programs can open AI files, but they may rasterize the image, meaning the vector data will be converted to a bitmap format.
NOTE: To open an Illustrator document in Photoshop, the file must first have PDF Content saved within the file. If it does not contain the PDF Content, then the graphic cannot be opened and will display a default message, stating, "This is an Adobe Illustrator file that was saved without PDF Content. To place or open this file in other applications, it should be re-saved from Adobe Illustrator with the "Create PDF Compatible File" option turned on."