An A-line skirt is a skirt that is fitted at the hips and gradually widens towards the hem, giving the impression of the shape of a capital letter A. The term is also used to describe dresses and coats with a similar shape.
A pencil skirt is a slim-fitting skirt with a straight, narrow cut. Generally, the hem falls to, or is just below, the knee and is tailored for a close fit. It is named for its shape: long and slim like a pencil.