The format of an artwork refers to the shape of its support and/or frame. When the artwork is a composition with several pieces, it refers to the format of the individual pieces. The format that I typically use are the following:

Landscape Rectangular format, with longer width than height. Artwork-162-web.jpg
Portrait Rectangular format, with longer height than width. Artwork-160-web.jpg
Square Square format. Artwork-119-web.jpg
Irregular The format is either irregular or variable. Artwork-200-web1.jpg