what is the exact usage of xmlns in xml, and html

Question

Does any one know what is the exact usage of xmlns in HTML, XML files?

Edit: Why do we need this namespace? What is its usage?

Answer 1

XML namespaces help contextualize elements an attributes, among other things. It also offers a precise identification for a particular element or attribute.

For instance, the element can be defined by anyone and have any meaning. However, the element within the http://www.w3.org/1999/xhtml namespace is unique and refers to the XHTML.

Namespaces also prove useful when dealing with homographs, when using multiple XML languages in a single file.