request,response,out,session,application,config,pageContext,page

request
This is the HttpServletRequest associated with the request, and lets you look at the request parameters (via getParameter), the request type (GET, POST, HEAD, etc.), and the incoming HTTP headers (cookies, Referer, etc.). Strictly speaking, request is allowed to be a subclass of ServletRequest other than HttpServletRequest, if the protocol in the request is something other than HTTP. This is almost never done in response
This is the HttpServletResponse associated with the response to the client. Note that, since the output stream (see out below) is buffered, it is legal to set HTTP status codes and response headers, even though this is not permitted in regular servlets once any output has been sent to the client.
out
This is the PrintWriter used to send output to the client. However, in order to make the response object (see the previous section) useful, this is a buffered version of PrintWriter called JspWriter. Note that you can adjust the buffer size, or even turn buffering off, through use of the buffer attribute of the page directive. This was discussed in Section 5. Also note that out is used almost exclusively in scriptlets, since JSP expressions automatically get placed in the output stream, and thus rarely need to refer to out explicitly.
session
This is the HttpSession object associated with the request. Recall that sessions are created automatically, so this variable is bound even if there was no incoming session reference. The one exception is if you use the session attribute of the page directive (see Section 5) to turn sessions off, in which case attempts to reference the session variable cause errors at the time the JSP page is translated into a servlet.
application
This is the ServletContext as obtained via getServletConfig().getContext().
config
This is the ServletConfig object for this page.
pageContext
JSP introduced a new class called PageContext to encapsulate use of server-specific features like higher performance JspWriters. The idea is that, if you access them through this class rather than directly, your code will still run on "regular" servlet/JSP engines.
page
This is simply a synonym for this, and is not very useful in Java. It was created as a placeholder for the time when the scripting language could be something other than Java.