在看FTP Upload And Download 相關書籍時，覺得卡卡的(觀念不通)。
於是我就上去StackFlow找了一下相關文章
目前在網上看到較優的解釋 ，這只是一個12歲的小孩寫的，觀念如此的清晰，羞愧了我
HttpWebRequest
is a concrete class that allows you to request in code any sort of
file over HTTP. You usually receive it as a stream of bytes. What you do with it
after that is up to your application.
HttpWebResponse
allows you to process the response from a web server that was
previously requested using HttpWebRequest.
WebRequest and WebResponse
are the abstract base classes that the HttpWebRequest and
HttpWebResponse inherit from. You can't create these directly. Other classes that
inherit from these include Ftp and File classes.
WebClient
I have always seen as a nice helper class that provides simpler ways to,
for example, download or upload a file from a web url. (eg DownloadFile and
DownloadString methods). I have heard that it actually uses HttpWebRequest /
HttpWebResponse behind the scenes for certain methods.
