Twill fabric is created by using a specific type of textile weave which creates a pattern of textured diagonal parallel ribs. Denim, for example, is a twill fabric.
Twill technically has a front & back side, with the front presenting the clearest view of the intended pattern. Twill is often used for durable & sturdy work clothing as stains are less noticeable on the textured fabric.
Twill is also wrinkle, air & water resistant due to its closely packed yarns.