A tag is a formatting code that is part of a document language, also known as a markup language, used to annotate a text in a way that is distinct from the text and provides instructions for how the text is to be displayed. There are various markup systems that have a large number of unique tags. The markup systems or languages include TeX, Scribe, GML, SGML, HTML, XML, and XHTML. HTML stands for HyperText Markup Language and it is used in the World Wide Web to create structure for text and documents and to create hyperlinks between documents, while XHTML (extensible HTML) combines HTML and XML. The input tag is an HTML and XHTML tag, and it is supported by Microsoft® Internet Explorer®, Mozilla® Firefox®, Opera®, Google® Chrome®, and Apple® Safari® browsers.
Many HTML and XHTML tags serve to format text that is on the page, but some create opportunities for user interaction, and the input tag is one of the latter: it is a key player in building forms. There is an important difference between how the input tag is used in HTML and how it is used in XHTML, however. In HTML the tag <input> is used, with the ellipses replaced by whatever content the tag holds, and has no end tag. In XHTML, the input tag requires a proper closing like this <input/>.