First add that using statement to import the necessary namespace for the XML stuff to work.
then do something like the following to load an XmlDocument object type from a string in memory.
// You can load your Xml string from anywhere you want.
string MyXmlString = "<doc>"+
" <nodes>"
" <node tag='node1' title='Hello world' description='some text here' />"
" <node tag='node2' title='Hello world' description='some text here' />"
" <node tag='node3' title='Hello world' description='some text here' />"
" <node tag='node4' title='Hello world' description='some text here' />"
" </nodes>"
"</doc>";
// Declare a new XmlDocument Instance.
XmlDocument doc = new XmlDocument();
/// now load XML from string.
doc.LoadXml(MyXmlString);
// Now go through our XML nodes one by one using foreach.
// This foreach loo below goes through elements of first node of the doc node.
// doc.DocumentElement doesn't mean necessarly our doc node above in the XML,
// but rather means the first root node of the document which is doc in our case here.
foreach (XmlNode node in doc.DocumentElement.ChildNodes[0].ChildNodes)
{
// you are going through nodes now :)
}