Block-level elements define the "big" structure of a Web page. Block elements begin on new lines, whereas inline (or text-level) elements do not. (You might wish to review the difference between the two.)
A classic example of a block element is the paragraph. You will observe that each paragraph on this page begins on a new line.
Blocks may contain other blocks, as well as inline elements and data (content).
A good procedure is to establish the block-level elements of your document first. These are the block-level elements covered in this tutorial: