Actually, those answers are wrong. They have elements and tags mixed up. An HTML document is built up out of elements and attributes. The Elements are all nested with the HTML element being the outer level / root element. Now each element may also contain certain attributes. A P element might contain the text node "Hello, World!" and also a style attribute. Attributes are in key / value pairs so the style attribute would be set to some value.
Tags create elements and attributes. The tag [font=monospace]<html lang=en">[/font] creates an HTML element with a lang attribute with a value of "en". By convention we would call that whole thing the HTML tag and it creates the HTML element and its attributes.
To make matters worse, tags are not the only way to create elements. In HTML the HTML, HEAD and BODY elements are required but the tags are not. If you omit the tags then the element is there nonetheless.