xmlns.com is an internet domain created for the purposes of simple Web namespace management. The rationale for registering xmlns.com was to secure a short, memorable domain suitable for naming concepts for use in RDF and XML vocabularies.
The intention is to provide persistent identifiers that can be used without fear of the domain changing hands and links being broken. Some xmlns.com vocabularies will simply be redirects to namespace information held at less persistent addresses (eg. schemata hosted on free Web provider sites). Other vocabularies can be hosted on the xmlns.com server itself.
The current namespace owner undertakes to service persistent http://xmlns.com/* URIs for as long as the Web exists and he's around to do so, and to pass ownership of xmlns.com on to other not-for-profit management arrangements when this proves too great a committment to honour singlehanded.
Currently there are two deployed namespaces at xmlns.com: FOAF and WOT. An experimental work-in-progress representing the wordnet vocabulary is available (sometimes), but should be considered unstable.
Comments/question to danbri.