Landscape and portrait are two different ways of looking at an image. In fact it is one of the first things you consider when you create an image with your camera.
The simplest way to consider this is by looking at these two images.
This image on the left is taking in landscape mode.
The top and bottom edges are longer than the two side edges of the image.
The photographer wanted to give the viewer a maximum view of the rolling waves.
The street image on the right has been taken in portrait mode.
The top and bottom edges are smaller than the two side edges.
By creating the image in this manner the photographer was able to give depth to the image leading you down the street to the building in the middle of the frame.