The significant differences that set a Sherwani and an Achkan apart are as follows:
Length: A Sherwani is slightly longer than an Achkan.
Fabric: Sherwanis are worn as decorative outer-coats at weddings and are thus made from heavy materials. Achkans are both made for formal and informal occasions, featuring lightweight materials with heavy embroidery in selective portions.
Flare: A Sherwani is more flared from the waist till bottom and Achkan is a rather tight-fitting.