A tag is a part of an HTML element, or rather, an element is the sum of a start-tag, optional content and optional end-tag.
e.g. The 'header' element is made up of the following:
<header> <-- Start tag of the 'header' element.
Text content, and other elements
</header> <-- Closing tag of the 'header' element.
Some elements like <br> don't need a closing tag (if the document is not intended to be XML-compliant) or text content.
Elements and their corresponding start tags can have attributes.
e.g. <img src="/opengraph.png" alt="">
In the above example, the 'img' element has two attributes in its start-tag: src and alt. The 'img' element doesn't have any contents and a closing-tag is not required in HTML5.