Type of files with extension AI is format developed by company known as Adobe Systems. Its full name is Adobe Illustrator Artwork. It is graphical format serving to create one-page pictures based on vectors. Application using this file format is called Adobe Illustrator and is also developed by the same company. It used to be marked as PGF format and it was compatible with PDF file format.
As it is written above, default program for creating and viewing files in this file format is Adobe Illustrator. But there are many alternatives. Next program, which can work with files saved in this file format is Adobe After Effects. That leads to use of these files in videos. To the known programs belongs program CorelDRAW that wasn´t created by Adobe Systems. He can also edit these files but there are some restrictions in editing. To free programs belongs also Inkscape, which can edit oled and even the newer version of this format based on PDF.
There are also only viewers developed by Adobe Systems and they are Adobe Reader and Adobe Photoshop. Both of them support only newer version supporting PDF format.