HyperText Markup Language provides a means to describe the structure of text-based information in a document — by denoting certain text as headings, paragraphs, lists, and so on — and to supplement that text with interactive forms, embedded images, and other objects. HTML is written in the form of labels, created by greater-than signs (>) and less-than signs (<). HTML can also describe, to some degree, the appearance and semantics of a document, and can include embedded scripting language code which can affect the behavior of web browsers and other HTML processors.

Tim Berners-Lee created the original HTML (and many associated protocols such as HTTP) to solve an immediate problem: the communication and dissemination of ongoing research among Berners-Lee and a group of his colleagues. His solution later combined with the emerging international and public internet to garner worldwide attention.

Early versions of HTML were defined with loose syntactic rules, which helped its adoption by those unfamiliar with web publishing. Web browsers commonly made assumptions about intent and proceeded with rendering of the page. Over time, as the use of authoring tools increased, the trend in the official standards has been to create an increasingly strict language syntax. However, browsers still continue to render pages that are far from valid HTML.

In Berners-Lee's book, Weaving the Web, several recurring themes are apparent:

• It is just as important to be able to edit the Web as browse it. Wikis are a step in this direction, although Berners-Lee considers them merely a shadow of the WYSIWYG functionality of his first browser.
• Computers can be used for background tasks that enable humans to work better in groups.
• Every aspect of the Internet should function as a Web, rather than a hierarchy. Notable current exceptions are the Domain Name System and the domain naming rules managed by ICANN.
• Computer scientists have a moral responsibility as well as a technical responsibility.

References: Wikipedia
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