Untitled

What is the purpose of partial classes?
public partial class MyClass : IDataErrorInfo
{
    partial void ValidateMyProperty(ref string error);

    public string this[string propertyName]
    {
        get
        {
            string error = String.Empty;
            if (propertyName == "MyProperty")
                ValidateMyProperty(ref error);

            return error;
        }
    }

    public string Error { get { return String.Empty; } }

    public int MyProperty { get; set; }
}

public partial class MyClass
{
    partial void ValidateMyProperty(ref string error)
    {
        if (MyProperty < 0)
            error = "MyProperty cannot be negative.";
    }
}

public partial class testClass {
  private int intMember;
  public string stringMember;
}

public partial class testClass {
  private int intAnotherInt;
  public string stringAnotherString;
}

public partial class testClass {
  private int intAnotherInt; // Our int from HelloWorld2.cs
  public string stringAnotherString; // Our string from HelloWorld2.cs
  private int intMember; // Our int from HelloWorld.cs
  public string stringMember; // Our string from HelloWorld.cs
}