DAVID BYRDEN
An XML element usually has a two-part name comprising a short 'local' part, such as "html", and a URL for its namespace, such as "http://www.w3.org/1999/xhtml".
The combined name is quite long and it would be inconvenient to write it in full each time.
So we use the 'local' part of the name in most of our XML document; but we link each and every usage to a single place in the document where we declare what the URL is.
The link is made by prefixing the local name with another name, like this: <og:title>. The prefix can be of any length and is followed by a colon. Usually these prefixes are made 2 or 3 characters long.
The declaration uses the special "xmlns" attribute, and a suffix that matches the prefix. For example: xmlns:og="http://ogp.me/ns#"
You can put these declarations in the start-tag of any element that surrounds the places where you refer to them. Usually they are put in the 'html' element, which surrounds the entire page.