What is the difference between block-level and inline elements?

Historically, block-level elements normally start on a new line and occupy the available horizontal space, while inline elements participate within a line of text. Common examples are div and p for block-level behavior, and span and a for inline behavior.

In modern CSS, the final layout behavior is controlled by the CSS display property, so an element's default display can be changed. HTML semantics and CSS layout behavior should therefore be treated as related but separate concerns.