The Hypertext Markup Language, most often referred to as simply HTML, is a standard markup language for web browsers. It was first practically implemented by Tim Berners-Lee in 1991, although it was already theoretically designed much earlier, in fact as early as 1980.
HTML uses elements – called tags – to describe the style and semantics of the content. They are also used to introduce non-textual elements. The tags are shortcodes surrounded by angle brackets.
The semantic tag would be eg. <h2> elements, describing level 2 headings (there are six levels of these), like the one above. This type of elements needs both an opening and a closing tag – </h2>. Other popular elements of this kind are <p> for paragraphs (like this one) or <a> for links (seen at the beginning).
The other type – content tag – includes elements such as <img /> which does not require a closing tag. It is used to include an image in the content.
There are some more HTML tags, that may combine both of these aims. One of these would be the <abbr> tag, which is used to explain abbreviations on hover.