Long ago when there were no shoulder pads or structure in the way people dressed the outfit of choice in the Asian region was the Angarkha. Different versions for different states and different regions. A complex version of a Kurta, the Angarkha was the matriarch of the wardrobe. With the British invasion is an influence came the tailoring techniques that were re-adapted in India to suit the newly inculcated western minds. About the same time the camera had been invented and portraiture was the best place to make a personal expression. Personal tailoring and heirloom jewellery were accessories to this fancy upper-class fad. As personal tailors adapted the ancient Angarkha in different avatars, the Angarkha started to evolve in to two directions.
The northern states that had fought the Moghul invasions chose to go with the formal & tapered version the Achkan. The states that leaned towards the Moghul ethos, refrained from this adaptation and embraced a modified version, namely the Sherwani. Hence the two looks, the Achkan & Sherwani became part of the two separate universes, visually and culturally, segregating the two philosophies, for ever.
Main difference between Sherwani and Achkan lies in their length, fabric and the massive flare from the waist. Sherwani is little longer than Achkan and is more flared from the waist till bottom and Achkan is more are trimmed & fitted to give a compact masculine look.
Achkan, the knee-length coat is commonly worn by the grooms during wedding ceremonies or other formal festive occasions as the Sherwani restricts its wearer, who needs to be tall for the flare and the length to fall in the right proportion.