HTML allows elements to be started and not terminated - if you miss a paragraph end tag, <P> then the paragraph will still be rendered. With XHTML the end tag must always be included, unless the element does not require any attributes.. For example:

<br /> is permissible for the HTML line-break (<BR>) in XHTML.

For an element that has no content, both starting and ending tags must be used as follows:

<p></p>