Denim is a sturdy cotton fabric that has a twill weave or plainweave. There are also other variations, including sateen which usually has a blend of cotton/polyester. Denim can come in different weights and even have spandex (stretch). Denim is one of our favorite fabrics and can make attractive jeans, jackets, skirts and dresses.
Twill is a diagonal weave structure. The twill weave gives the fabric more durability and body. Twill can be found in top and bottom weight fabrics.