A single element will be placed in the top-left corner of the screen. As more elements are added, they are placed to the right of each preceding element until there is not space left. Then they move below to to begin again at the left, just like word wrap for text in a Word document.
Some elements in xhtml e.g <p></p> are block-level elements which will always take a new line so that they do not sit to the right of the preceding element.
In CSS any element can be defined as a block-level element
or can be floated to make it follow the flow.