Landscape orientation means a painting is meant to be displayed horizontally, and portrait orientation means that a painting is mean to be displayed vertically. These terms are derived from the typical subjects of paintings that call for the different types of orientation.
Landscape orientation is called such because it allows for a greater view of a landscape and the horizon. Portrait orientation is called such because it allows for a greater view of a person's face, shoulders and body in a painting. In modern times, these terms have been adopted for use in photography and the printing industry. In photography, the different orientations are generally used for the same subjects as they are in paintings.