What is the trace in ASP.NET

ASP.NET introduces new functionality that allows you to view
diagnostic information about a single request for an ASP.NET page
simply by enabling it for your page or application. Called tracing,
this feature also allows you to write debug statements directly
in your code without having to remove them from your
application when it is deployed to production servers.
You can write variables or structures in a page, assert whether a
condition is met, or simply trace through the execution path of
your page or application.

In order for these messages and other tracing information to be
gathered and displayed, you must enable tracing for the page or application.
When you enable tracing, two things occur:

ASP.NET appends a series of diagnostic information tables
immediately following the page's output. The information is also
sent to a trace viewer application
(if you have enabled tracing for the application).
ASP.NET displays your custom diagnostic messages in the
Trace Information table of the appended performance data.
Diagnostic information and tracing messages that you specify
are appended to the output of the page that is sent to the requesting browser.
Optionally, you can view this information from a separate
trace viewer (Trace.axd) that displays trace information for every
page in a given application. This information can help you to
clarify errors or undesired results as ASP.NET processes a page request.

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.