Twill fabrics have a weft thread that runs over and under multiple warp threads (as opposed to a plain weave where the weft crosses a single warp thread at a time). This can create interesting patterns like a herringbone, houndstooth or a simple, diagonal rib. Twill is a beautiful, interesting and diverse weave: different threads yield dramatically different results.
There’s a decidedly luxurious, elegant look to a fine twill shirt with its silky diagonal texture, lustrous sheen and gentle drape. (The ‘drape’ is the way a fabric hangs: so whilst poplin has its sharp, crisp look, twill hangs softly.) The fineness of twill derives from the fineness of the thread and the tightness of the weave. The fabric itself is more substantial than poplin, and highly durable.
Being so soft, a twill shirt is easier to iron and more resistant to wrinkling.