The HTML markup language is made up of lots of different kinds of "tags". Most tags follow this format:
A simple example:
<p>This is a paragraph.</p>
Some HTML tags (like img and br) don't have their own content. These are known as self closing tags or empty tags. They look like this:
A simple example:
<br />
The br tag inserts a line break (not a paragraph break). This tag has no content, so it is self closing.
With thanks to Alissa Miller.