Twill is a type of fabric woven with a pattern of diagonal parallel ribs.
It is made by passing the weft threads over one warp thread and then under two or more warp threads, over one and under two or more, and so on, with a "step" or offset between rows that creates the characteristic diagonal pattern. Examples of twill fabric are chino, denim, gabardine, tweed and serge.
Twill, from a practical standpoint, is a warm weather material, less well suited for cold weather. It also tends to drape well.