Meaning and definition of xmlns=""

In wpf and silverlight when using xaml this has to be declared at the begginning of the xaml file

 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions" 
    xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity" 

but,where can i find what does every entry means?, as far as i Know they are similar to the using in c# or imports in VB, but where is the definition of the class or "namespace"?

Thanks a lot!!!

xmlns=... is a namespace declaration. See XML Namespaces: http://en.wikipedia.org/wiki/XML_namespace