Tracing is a refined process for outputting page-level information.When tracing is enabled you automatically get information on the ASP.NET Web page. Information like:

Request Details – Session Id; Request time, type, and encoding; status code, etc.
Trace Information – Page-level ASP.NET messages that you specify via Trace.Write and Trace.Warn.
Control Tree – A listing of the Web controls on theASP.NET Web page,and how they relate to one another.
Cookies Collection – A listing of all of the cookies.
Headers Collection – A listing of all of the HTTP headers.
Server Variables – A listing of all of the server variables.

Trace statements are processed and displayed only when tracing is enabled. You can control whether tracing is displayed to a page, to the trace viewer, or both.