structure of HttpServletRequest object and the request url mechanism

[lovekang]

API says about getInputStream() of HttpServletRequest is like this.
Retrieves the body of the request as binary data using a ServletInputStream.
then request is composed of headers and body.
and body is composed of parameters and their values.
and headers can be viewed with getHeaders(java.lang.String name)
and body can be viewed with getInputStream().
right?
tell me more about request objects structure.
and I want to know what information is sent to the server or servlet when a user request a url.
Thanks in advance.