HTML is a markup language that allows for defining a webpage.
The web page as the user interface consists of the controls
in a hierarchy tree ie. there are controls of "parent" and "child" types.

In the above example, we have presented the page consisting of the table.
The table contains three rows. Each row contains cells.
The cells contain labels, fields for text input and a button.

Each element of the page is described using the so-called tags.
Every opening tag has its closing counterpart eg. <table> ... </table>
Between the opening and closing tags are child elements.