An Adobe Illustrator vector format file. Files with the extension .ai are often used for artwork and illustrations as users can resize them without distortion. It is composed of several layers with objects and text on each layer. Users can import Illustrator files into many other programs like InDesign, Acrobat, Photoshop etc. If you design or print your t-shirts, you can create your designs in Illustrator and apply them to your t-shirt using a heat press.