A sheath/column is a narrow shape that flows straight down from neckline to hem. The term Sheath and Column can pretty much be used interchangeably and both run vertically from the shoulders down to the floor and the hem does not flair out like you may see in an A-Line, Empire fit and flare, or Ball Gown.
These dresses tend to fit close to the body but not as tight as a fitted silhouette. Typically, these dresses work well for slimmer figures whether you are tall or short, but can also be worn by fuller figures often with a belt to emphasize the waist a bit more. For petite brides, a sheath/ column can elongate the figure giving more height to your look.