Normally, if you click something in an overview you will go to another page. It doesn't have to be an article, but usually it is. The title and the lead text is shown on the overview, but not in the article itself.
Now, if the body field is empty, there won't be any link. In the overview, the article isn't clickable.
If the article contains more than one image, those images need to go somewhere. So then, in the overview, a click-through gallery is created. Like so:
This works for all different kinds of overviews. Note how the title isn't displayed, but instead the description of the image. That way you can have different descriptions for each image.