HTML (Hypertext Markup Language) is the standard markup language used to create web pages. Here are some key HTML concepts:

1. *Tags*: HTML uses tags (elements) to define content and structure. Tags are surrounded by angle brackets (<>) and usually come in pairs (opening and closing).
2. *Elements*: HTML elements represent different parts of a web page, such as headings, paragraphs, images, links, forms, tables, and more.
3. *Attributes*: HTML elements can have attributes, which provide additional information about the element, such as the source of an image or the destination of a link.
4. *Semantic meaning*: HTML elements have semantic meaning, which means they convey meaning to search engines, screen readers, and other devices, improving accessibility and SEO.
5. *Structural elements*: HTML has structural elements like <header>, <nav>, <main>, <section>, <article>, <aside>, <footer>, etc., which define the structure of a web page.
6. *Content elements*: HTML has content elements like <p>, <img>, <ul>, <ol>, <li>, <table>, <td>, etc., which represent different types of content.
7. *Metadata*: HTML has metadata elements like <title>, <meta>, <link>, which provide information about the document, such as title, description, and links to external resources.

Understanding these HTML concepts is essential for building websites, web applications, and mobile applications.