Block elements are HTML elements that take up the full width of their parent container, creating a new line before and after themselves. Common examples of block elements include div, h1, p, and section. They are typically used to structure the layout of a webpage, allowing for clear separation of content. These elements can contain inline elements, such as span or a, but they cannot be nested within inline elements. Block elements are essential for organizing content in a way that is visually appealing and easy to read, making them a fundamental part of web design.