Each part of document is referred to as an element. In HTML, these elements are identified through the use of tags.
The user-agent (or client) is responsible for interpreting the tags and displaying the associated information in an organized and essentially standard fashion. Unfortunately, numerous differences in interpretation exist.
All HTML tags are identified by enclosing them in angle brackets "<" and ">". Information within these brackets is for use by the browser and is not displayed in the document window.
Many HTML tags exist in pairs: a start tag, followed by the contents of the specific structure, then an end tag. For example:
<HTML>... an HTML document goes here! ...
</HTML>
The start tag identifies the specific structure element, and the end tag contains the same name for proper matching of tags preceded by a slash "/", which denotes the end of the structure.
Most tags which do not identify structure elements or affect a specific range, do not have an end tag.
Any tags not supported by a browser are simply ignored. This means the browser will not "crash" the users system when it encounters some unknown element, but it also means that the contents of the tag will be treated differently than intended.