Sheath dress: what makes it special?

Dear users, dear authors,
What is a sheath dress, and when do you wear one?

A sheath dress makes for a nice figure if you’re slim enough for it; otherwise, unfortunately, it’s just a disaster if you’re too wide and squeeze yourself into a dress that doesn’t suit you at all. Sadly, you see this happening again and again if you walk through town with your eyes open. Otherwise, a sheath dress is a dress that always makes you look appropriately dressed.

Best, Inge