What is the difference between the article and section elements in HTML?

article is a self-contained, independent piece of content that can be distributed or reused outside the main context of a page. Examples include a blog post, news item, comment, or widget.

section is a thematic group of content. It can be a division on a page (e.g., "Introduction", "Contents", "Contacts") or a group of related paragraphs, images, etc. section often has a heading.

Main difference: article is a content unit that makes sense on its own, while section is a structural part of a page or document.

Example of usage:

<article>
  <h2>Article Title</h2>
  <p>Main text of the article...</p>
  <section>  <!-- Subsection within the article -->
    <h3>Comments</h3>
    <article> <!-- A separate comment as a self-contained element -->
      <p>Comment text.</p>
    </article>
  </section>
</article>

<section> <!-- A separate section on the page, not related to the previous article -->
  <h2>Section with additional information</h2>
  <p>Additional information text...</p>
</section>