So this one is not mine, but this is a great, brief explanation of the various methods of posting via HttpWebRequest:

http://www.wintellect.com/CS/blogs/jeffreyr/archive/2009/02/08/httpwebrequest-its-request-stream-and-sending-data-in-chunks.aspx

Great information if you need to send large amounts of data over a direct http connection (like I need to do).