People use the words coffin and casket interchangeably when talking about funeral arrangements, but morticians know there's more than just a price difference between the two-the difference is in the shape and function.
Coffins are the octagonal burial containers made wider at the shoulder and tapered at the feet to fit the body, and they're used pretty much exclusively for burial.
But by the mid-19th century undertakers had become funeral directors, and they needed something more luxurious and showy than a coffin for viewing purposes- so the casket was created.
Caskets are lined and typically rectangular shaped burial containers with hinged lids and decorative elements, and nowadays caskets are often custom made to suit the deceased to a T.
-Via mental_floss