The correlation ID is the unique identifier for the request, not for the error itself.
In simple cases, it can be used to quickly search the logs to get a more detailed error message (e.g with a stack trace) than what was shown to the user. You can either search the trace log files or the logging database with that correlation ID and you'll get every log message related to that request. In more complex cases, the error might have been caused by something that happened earlier in the page's life cycle, so being able to narrow your search will get you the information you need faster.
The correlation ID is written to the end of each line in the trace log files, which are by default stored in
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\LOGSlike Tom Halladay said. Alternatively, you can search the
ULSTraceLog view, which has a separate field for the correlation ID.