GeoNetwork is a web application that allows you to maintain a geographic referenced metadata catalogue. This means, a search portal that allows you to view metadata combined with maps. It strictly follows different standards for metadata, from Inspire to OGC. This has allowed GeoNetwork to expand to a lot of organizations, like the swiss geoportal or the brasilian one, not forgetting the New zealander.
It is deployed inside a java application container (like tomcat or jetty), working over the Jeeves framework. Jeeves is based on XSTL transformations that allows a simple quick development (and powerfull) of interfaces, for humans or machines (XML). This makes metadata from GeoNetwork to be easily accesible by different platforms.