A portrait is a picture of a person. It could be a photograph, a painting, a drawing. In desktop publishing, portrait usually refers to the orientation of a piece of paper. Typically a traditional portrait is a rectangle that is taller than it is wide, thus a piece of paper, a greeting card, a poster, etc. that is oriented so that the shorter sides are the top and bottom is in portrait orientation.
Although they can be in the landscape orientation, book and magazine pages, posters, and greeting cards are most frequently in portrait mode. The default setting for most desktop printers is to print in portrait mode so if you want to print in landscape mode, be sure to adjust the printer options before printing.