The larger the aperture opening (smaller number) the less depth of field you have. So when you see those great portraits with just the person in focus and the background blurred, that is done by controlling the aperture. When you see landscape photographs with the whole scene amazingly sharp, that is also done by choosing the appropriate aperture.
Unlike some other types of photography, landscape photographs should include crisp focus throughout, hence the frequent use of tripods or other stabilizing devices. Although I prefer nature scenes, landscape photography is not limited to such. It also includes cityscapes, and can include sunrise and sunset photographs, people, and other natural components such as trees, mountains, and clouds.