The Block module treats every content element on the page as a "block" as opposed to a disorganized free flowing text. This new way of thinking gives every element a controllable and predictable behavior. Even a paragraph with a picture is treated as a block of text followed by a block of image.
The block paradigm provides a virtual boundary between other elements in a selectable and clickable form allowing you to make changes using a visual editor. As blocks, you can drag them to organize, delete, clone and style them. You can compose smaller blocks into larger blocks to create complex content.