1 HTML elements are comprised of three major parts: the opening tag, the content, and the closing tag. As you will learn, there are infinite combinations of HTML tags/elements, including those used for forms, images, and lists. Everything displayed on an web page requires the use of a tag or two.
<tag>Content</tag> <p>This text will be rendered like a paragraph.</p>
Tags should always be written in lowercase letters if you plan on publishing the page
2 There are a few elements that do not require formal closing tags. In a way, they still have the 3 parts (opening/closing and content), but they are consolidated into one tag.
<img src="../mypic.jpg" /> -- Image Tag <br /> -- Line Break Tag <input type="text" size="12" /> -- Input Field
In a way, they still have the 3 parts (opening/closing and content), but they are consolidated into one tag. The reason for this is that these tags do not really require any content to be placed within them, but sometimes may require attributes such as source URLs for images.
Reference:
http://www.tizag.com/htmlT/tags.php