HTML Tag: Symbols used to define the structure and formatting of an HTML document. This includes the structure of the document with <head> and <body> tags, layout of the document with tags like <table> or <div> and formatting of text with tags like <b> for bold and <i> for italic. Tags have a starting form like <this> and an ending like </this> and can have a number of attributes like <tag attribute="value"> this.
Tags can be written by hand, generated by a formatting program or created by a WYSIWYG (what you see is what you get) editor such as Microsoft Frontpage. HTML tags are used in Ruby usually in conjunction with embedded Ruby or an HTML or XML builder interface.