some text
This is a <div>. By default a <div> has the "display" CSS property set to "block". Block elements are rectangular in shape, and take as much width as they can. They also break the flow of the page.
some text This is a <span> By default a <span> has the "display" CSS property set to "inline". Inline elements take the shape of whatever is inside of them. They stretch to fit the width of the content inside of them, and they do not break the flow of the page. some text

Common "block" tags:

Common "inline" tags: