A debug log records database operations, system processes, and errors that occur when executing a transaction or while running unit tests. The system generates a debug log for a user every time that user executes a transaction that is included in the filter criteria. (You need to add that user to the debug log).
http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_debugging_debug_log.htm
Thanks
Utsav
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