Most HTML elements have a start tag and an end tag that indicate where the element begins and where it ends. There is a group of elements that are exceptions to this rule. These elements are called empty or void and only have a start tag since they can't have any content. They must not have an end tag in HTML.
The void elements in HTML 4.01/XHTML 1.0 Strict are area, base, br, col, hr, img, input, link, meta, and param. HTML5 currently adds command, keygen, and source to that list.