The names come from paintings. Visualize a painting of a landscape: it will
be wider than it is tall, whereas a portrait painting is taller than it is
wide. Now take your sheet of paper and hold it upright, like a letter:
that's portrait. If you rotate it a quarter-turn, that will be landscape;
you might use this orientation for a chart or a wide table.
Note that when you print portrait or landscape, you insert the paper into
your printer the same way, and the print on your screen still goes from side
to side (you don't have to turn your head sideways to read it). This
confuses some people, but Word works with the printer to rotate the print on
the page so that it runs in a different direction.