LINQ and some difference between XDocument and XElement

app.config
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  
  <MyConfig>
    <Products>
      <ProductItem Property1="xyz" Property2="abc"/>
      <ProductItem Property1="sddf" Property2="ljd"/>
      <ProductItem Property1="oiu" Property2="sldj"/>
    </Products>
    <Store>
      <StoreItem Property1="xyz" Property2="abc"/>
      <StoreItem Property1="sddf" Property2="ljd"/>
    </Store>
    <File Name="HelloWorld" Type="doc"/>
    <ErrorCode Message="This is incorrect" Length="20"/>
  </MyConfig>
  
  <configSections>
  </configSections>
  
  <connectionStrings>
    <add name="WindowsFormsApplication1.Properties.Settings.Setting"
        connectionString="Data Source=SHA-fifei-vista;Initial Catalog=test;Integrated Security=True"
        providerName="System.Data.SqlClient" />
  </connectionStrings>
  
</configuration>

Winform code:

            XElement config  =  XElement.Load(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);
             foreach  (XElement xe  in  config.Elements())
                textBox1.Text  +=  (xe.Name.ToString()  +  Environment.NewLine);

            XDocument xd  =  XDocument.Load(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);
             foreach  (XElement xe  in  xd.Elements())
                textBox1.Text  +=  (xe.Name.ToString()  +  Environment.NewLine);

Output:
MyConfig
configSections
connectionStrings
configuration

some other XLINQ samples
            XElement doc = XElement.Load(@"D:\sample.xml");

            XDocument contactsDoc = new XDocument(
                new XDeclaration("1.0", "utf-8", "yes"),
                new XComment("LINQ to XML Contacts XML Example"),
                new XProcessingInstruction("MyApp", "123-44-4444"),
                    new XElement("contacts",
                        new XElement("contact",
                            new XElement("name", "Patrick Hines"),
                            new XElement("phone", "206-555-0144"),
                            new XElement("address",
                                new XElement("street1", "123 Main St"),
                                new XElement("city", "Mercer Island"),
                                new XElement("state", "WA"),
                                new XElement("postal", "68042")
                                ))));

            XElement contactsDoc2 = XElement.Parse(@"<contacts>
                <contact>
                    <name>Patrick Hines</name> 
                    <phone>206-555-0144</phone> 
                    <address>
                        <street1>123 Main St</street1> 
                        <city>Mercer Island</city> 
                        <state>WA</state> 
                        <postal>68042</postal> 
                    </address>
                </contact>
            </contacts>");

            IEnumerable<XElement> contacts = contactsDoc.Elements("contact");
            contactsDoc.Save(@"C:\sample.xml");
            contactsDoc2.Save(@"C:\sample2.xml");
            doc.Save(@"C:\sampleResult.xml");

There, u can c the differences between XElement and XDocument :)
In the past, I wrote some XML operation tutorials in non-LINQ time: http://hi.baidu.com/feishunji/blog/category/Xml

Marius Bancila has also contributed some good introduction articles of LINQ:
Introduction to LINQ, Part 1: LINQ to Objects
Introduction to LINQ, Part 2: LINQ to XML
Introduction to LINQ, Part 3: LINQ to SQL