I'm curious about this code snippet:

public static class XNAExtensions
{
    /// <summary>
    /// Write a Point
    /// </summary>
    public static void Write(this NetOutgoingMessage message, Point value)
    {
        message.Write(value.X);
        message.Write(value.Y);
    }
    // ...
};

What does the this keyword mean next to the parameter type? I can't seem to find any information about it anywhere, even in the C# specification.