HTML has various elements for different purposes. Each of them have a unique notation and they are defined by start tag, some content and close tag.
HTML Element normally consists of a start tag and end or close tag with some content inside them.
<tagname>Content will be here</tagname>
The above syntax combinedly known as HTML Element. For example:
<h1>Heading</h1>
<p>Paragraph</p>
The above tags are known as the conatiner tags, as they contains some content and have a ending tag.
Empty Tags
In HTML their are empty tags also. Empty tags are those tags which does not contain any content and no end tag. For example:
<br> (This tag is used for line break in HTML)
<hr> (This tag is used to create a horizontal line in HTML)
Nested HTML
In HTML elements can be nested(Elements within element). Every HTML document has nested HTML.
For example:
<!DOCTYPE html>
<html>
<head>
<title>Page Title</title>
</head>
<body>
<h1>Page Heading</h1>
<p>Paragraph</p>
</body>
</html>
In the above example <html> defines the whole document with <html> as starting tag and </html> as ending tag and it contains <head> which further contains <title> element and <body> elements which further contains <h1> and <p> elements.
All the above stated elements have a starting tag and a closing tag.