Namespaces are used to distinguish elements or attributes with the same name, e.g. you can distinguish an <excel:table>
from a <furniture:table>
even in an XML which is an Excel table of furtniture tables.
The namespace can be anything, it should just be unique. Many people use their own URL because the probability is higher that noone else uses it. In many cases, visiting the URL isn't even possible.
However, there are some namespaces you should avoid anything starting with x-schema:
, because that will be interpreted as a Microsoft XDR schema, some obsolete Microsoft schema stuff.
XML like
<myxml xmlns="irldev_windows8_phone" />
is well-formed XML with your own namespace.